Merge.
theory Proof
imports Main
begin
chapter {* Proofs \label{ch:proofs} *}
text {*
Proof commands perform transitions of Isar/VM machine
configurations, which are block-structured, consisting of a stack of
nodes with three main components: logical proof context, current
facts, and open goals. Isar/VM transitions are typed according to
the following three different modes of operation:
\begin{description}
\item @{text "proof(prove)"} means that a new goal has just been
stated that is now to be \emph{proven}; the next command may refine
it by some proof method, and enter a sub-proof to establish the
actual result.
\item @{text "proof(state)"} is like a nested theory mode: the
context may be augmented by \emph{stating} additional assumptions,
intermediate results etc.
\item @{text "proof(chain)"} is intermediate between @{text
"proof(state)"} and @{text "proof(prove)"}: existing facts (i.e.\
the contents of the special ``@{fact_ref this}'' register) have been
just picked up in order to be used when refining the goal claimed
next.
\end{description}
The proof mode indicator may be understood as an instruction to the
writer, telling what kind of operation may be performed next. The
corresponding typings of proof commands restricts the shape of
well-formed proof texts to particular command sequences. So dynamic
arrangements of commands eventually turn out as static texts of a
certain structure.
\Appref{ap:refcard} gives a simplified grammar of the (extensible)
language emerging that way from the different types of proof
commands. The main ideas of the overall Isar framework are
explained in \chref{ch:isar-framework}.
*}
section {* Proof structure *}
subsection {* Blocks *}
text {*
\begin{matharray}{rcl}
@{command_def "next"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "{"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "}"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
\end{matharray}
While Isar is inherently block-structured, opening and closing
blocks is mostly handled rather casually, with little explicit
user-intervention. Any local goal statement automatically opens
\emph{two} internal blocks, which are closed again when concluding
the sub-proof (by @{command "qed"} etc.). Sections of different
context within a sub-proof may be switched via @{command "next"},
which is just a single block-close followed by block-open again.
The effect of @{command "next"} is to reset the local proof context;
there is no goal focus involved here!
For slightly more advanced applications, there are explicit block
parentheses as well. These typically achieve a stronger forward
style of reasoning.
\begin{description}
\item @{command "next"} switches to a fresh block within a
sub-proof, resetting the local context to the initial one.
\item @{command "{"} and @{command "}"} explicitly open and close
blocks. Any current facts pass through ``@{command "{"}''
unchanged, while ``@{command "}"}'' causes any result to be
\emph{exported} into the enclosing context. Thus fixed variables
are generalized, assumptions discharged, and local definitions
unfolded (cf.\ \secref{sec:proof-context}). There is no difference
of @{command "assume"} and @{command "presume"} in this mode of
forward reasoning --- in contrast to plain backward reasoning with
the result exported at @{command "show"} time.
\end{description}
*}
subsection {* Omitting proofs *}
text {*
\begin{matharray}{rcl}
@{command_def "oops"} & : & @{text "proof \<rightarrow> local_theory | theory"} \\
\end{matharray}
The @{command "oops"} command discontinues the current proof
attempt, while considering the partial proof text as properly
processed. This is conceptually quite different from ``faking''
actual proofs via @{command_ref "sorry"} (see
\secref{sec:proof-steps}): @{command "oops"} does not observe the
proof structure at all, but goes back right to the theory level.
Furthermore, @{command "oops"} does not produce any result theorem
--- there is no intended claim to be able to complete the proof
anyhow.
A typical application of @{command "oops"} is to explain Isar proofs
\emph{within} the system itself, in conjunction with the document
preparation tools of Isabelle described in \chref{ch:document-prep}.
Thus partial or even wrong proof attempts can be discussed in a
logically sound manner. Note that the Isabelle {\LaTeX} macros can
be easily adapted to print something like ``@{text "\<dots>"}'' instead of
the keyword ``@{command "oops"}''.
\medskip The @{command "oops"} command is undo-able, unlike
@{command_ref "kill"} (see \secref{sec:history}). The effect is to
get back to the theory just before the opening of the proof.
*}
section {* Statements *}
subsection {* Context elements \label{sec:proof-context} *}
text {*
\begin{matharray}{rcl}
@{command_def "fix"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "assume"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "presume"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "def"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
\end{matharray}
The logical proof context consists of fixed variables and
assumptions. The former closely correspond to Skolem constants, or
meta-level universal quantification as provided by the Isabelle/Pure
logical framework. Introducing some \emph{arbitrary, but fixed}
variable via ``@{command "fix"}~@{text x}'' results in a local value
that may be used in the subsequent proof as any other variable or
constant. Furthermore, any result @{text "\<turnstile> \<phi>[x]"} exported from
the context will be universally closed wrt.\ @{text x} at the
outermost level: @{text "\<turnstile> \<And>x. \<phi>[x]"} (this is expressed in normal
form using Isabelle's meta-variables).
Similarly, introducing some assumption @{text \<chi>} has two effects.
On the one hand, a local theorem is created that may be used as a
fact in subsequent proof steps. On the other hand, any result
@{text "\<chi> \<turnstile> \<phi>"} exported from the context becomes conditional wrt.\
the assumption: @{text "\<turnstile> \<chi> \<Longrightarrow> \<phi>"}. Thus, solving an enclosing goal
using such a result would basically introduce a new subgoal stemming
from the assumption. How this situation is handled depends on the
version of assumption command used: while @{command "assume"}
insists on solving the subgoal by unification with some premise of
the goal, @{command "presume"} leaves the subgoal unchanged in order
to be proved later by the user.
Local definitions, introduced by ``@{command "def"}~@{text "x \<equiv>
t"}'', are achieved by combining ``@{command "fix"}~@{text x}'' with
another version of assumption that causes any hypothetical equation
@{text "x \<equiv> t"} to be eliminated by the reflexivity rule. Thus,
exporting some result @{text "x \<equiv> t \<turnstile> \<phi>[x]"} yields @{text "\<turnstile>
\<phi>[t]"}.
\begin{rail}
'fix' (vars + 'and')
;
('assume' | 'presume') (props + 'and')
;
'def' (def + 'and')
;
def: thmdecl? \\ name ('==' | equiv) term termpat?
;
\end{rail}
\begin{description}
\item @{command "fix"}~@{text x} introduces a local variable @{text
x} that is \emph{arbitrary, but fixed.}
\item @{command "assume"}~@{text "a: \<phi>"} and @{command
"presume"}~@{text "a: \<phi>"} introduce a local fact @{text "\<phi> \<turnstile> \<phi>"} by
assumption. Subsequent results applied to an enclosing goal (e.g.\
by @{command_ref "show"}) are handled as follows: @{command
"assume"} expects to be able to unify with existing premises in the
goal, while @{command "presume"} leaves @{text \<phi>} as new subgoals.
Several lists of assumptions may be given (separated by
@{keyword_ref "and"}; the resulting list of current facts consists
of all of these concatenated.
\item @{command "def"}~@{text "x \<equiv> t"} introduces a local
(non-polymorphic) definition. In results exported from the context,
@{text x} is replaced by @{text t}. Basically, ``@{command
"def"}~@{text "x \<equiv> t"}'' abbreviates ``@{command "fix"}~@{text
x}~@{command "assume"}~@{text "x \<equiv> t"}'', with the resulting
hypothetical equation solved by reflexivity.
The default name for the definitional equation is @{text x_def}.
Several simultaneous definitions may be given at the same time.
\end{description}
The special name @{fact_ref prems} refers to all assumptions of the
current context as a list of theorems. This feature should be used
with great care! It is better avoided in final proof texts.
*}
subsection {* Term abbreviations \label{sec:term-abbrev} *}
text {*
\begin{matharray}{rcl}
@{command_def "let"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{keyword_def "is"} & : & syntax \\
\end{matharray}
Abbreviations may be either bound by explicit @{command
"let"}~@{text "p \<equiv> t"} statements, or by annotating assumptions or
goal statements with a list of patterns ``@{text "(\<IS> p\<^sub>1 \<dots>
p\<^sub>n)"}''. In both cases, higher-order matching is invoked to
bind extra-logical term variables, which may be either named
schematic variables of the form @{text ?x}, or nameless dummies
``@{variable _}'' (underscore). Note that in the @{command "let"}
form the patterns occur on the left-hand side, while the @{keyword
"is"} patterns are in postfix position.
Polymorphism of term bindings is handled in Hindley-Milner style,
similar to ML. Type variables referring to local assumptions or
open goal statements are \emph{fixed}, while those of finished
results or bound by @{command "let"} may occur in \emph{arbitrary}
instances later. Even though actual polymorphism should be rarely
used in practice, this mechanism is essential to achieve proper
incremental type-inference, as the user proceeds to build up the
Isar proof text from left to right.
\medskip Term abbreviations are quite different from local
definitions as introduced via @{command "def"} (see
\secref{sec:proof-context}). The latter are visible within the
logic as actual equations, while abbreviations disappear during the
input process just after type checking. Also note that @{command
"def"} does not support polymorphism.
\begin{rail}
'let' ((term + 'and') '=' term + 'and')
;
\end{rail}
The syntax of @{keyword "is"} patterns follows \railnonterm{termpat}
or \railnonterm{proppat} (see \secref{sec:term-decls}).
\begin{description}
\item @{command "let"}~@{text "p\<^sub>1 = t\<^sub>1 \<AND> \<dots> p\<^sub>n = t\<^sub>n"} binds any
text variables in patterns @{text "p\<^sub>1, \<dots>, p\<^sub>n"} by simultaneous
higher-order matching against terms @{text "t\<^sub>1, \<dots>, t\<^sub>n"}.
\item @{text "(\<IS> p\<^sub>1 \<dots> p\<^sub>n)"} resembles @{command "let"}, but
matches @{text "p\<^sub>1, \<dots>, p\<^sub>n"} against the preceding statement. Also
note that @{keyword "is"} is not a separate command, but part of
others (such as @{command "assume"}, @{command "have"} etc.).
\end{description}
Some \emph{implicit} term abbreviations\index{term abbreviations}
for goals and facts are available as well. For any open goal,
@{variable_ref thesis} refers to its object-level statement,
abstracted over any meta-level parameters (if present). Likewise,
@{variable_ref this} is bound for fact statements resulting from
assumptions or finished goals. In case @{variable this} refers to
an object-logic statement that is an application @{text "f t"}, then
@{text t} is bound to the special text variable ``@{variable "\<dots>"}''
(three dots). The canonical application of this convenience are
calculational proofs (see \secref{sec:calculation}).
*}
subsection {* Facts and forward chaining *}
text {*
\begin{matharray}{rcl}
@{command_def "note"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "then"} & : & @{text "proof(state) \<rightarrow> proof(chain)"} \\
@{command_def "from"} & : & @{text "proof(state) \<rightarrow> proof(chain)"} \\
@{command_def "with"} & : & @{text "proof(state) \<rightarrow> proof(chain)"} \\
@{command_def "using"} & : & @{text "proof(prove) \<rightarrow> proof(prove)"} \\
@{command_def "unfolding"} & : & @{text "proof(prove) \<rightarrow> proof(prove)"} \\
\end{matharray}
New facts are established either by assumption or proof of local
statements. Any fact will usually be involved in further proofs,
either as explicit arguments of proof methods, or when forward
chaining towards the next goal via @{command "then"} (and variants);
@{command "from"} and @{command "with"} are composite forms
involving @{command "note"}. The @{command "using"} elements
augments the collection of used facts \emph{after} a goal has been
stated. Note that the special theorem name @{fact_ref this} refers
to the most recently established facts, but only \emph{before}
issuing a follow-up claim.
\begin{rail}
'note' (thmdef? thmrefs + 'and')
;
('from' | 'with' | 'using' | 'unfolding') (thmrefs + 'and')
;
\end{rail}
\begin{description}
\item @{command "note"}~@{text "a = b\<^sub>1 \<dots> b\<^sub>n"} recalls existing facts
@{text "b\<^sub>1, \<dots>, b\<^sub>n"}, binding the result as @{text a}. Note that
attributes may be involved as well, both on the left and right hand
sides.
\item @{command "then"} indicates forward chaining by the current
facts in order to establish the goal to be claimed next. The
initial proof method invoked to refine that will be offered the
facts to do ``anything appropriate'' (see also
\secref{sec:proof-steps}). For example, method @{method_ref rule}
(see \secref{sec:pure-meth-att}) would typically do an elimination
rather than an introduction. Automatic methods usually insert the
facts into the goal state before operation. This provides a simple
scheme to control relevance of facts in automated proof search.
\item @{command "from"}~@{text b} abbreviates ``@{command
"note"}~@{text b}~@{command "then"}''; thus @{command "then"} is
equivalent to ``@{command "from"}~@{text this}''.
\item @{command "with"}~@{text "b\<^sub>1 \<dots> b\<^sub>n"} abbreviates ``@{command
"from"}~@{text "b\<^sub>1 \<dots> b\<^sub>n \<AND> this"}''; thus the forward chaining
is from earlier facts together with the current ones.
\item @{command "using"}~@{text "b\<^sub>1 \<dots> b\<^sub>n"} augments the facts being
currently indicated for use by a subsequent refinement step (such as
@{command_ref "apply"} or @{command_ref "proof"}).
\item @{command "unfolding"}~@{text "b\<^sub>1 \<dots> b\<^sub>n"} is structurally
similar to @{command "using"}, but unfolds definitional equations
@{text "b\<^sub>1, \<dots> b\<^sub>n"} throughout the goal state and facts.
\end{description}
Forward chaining with an empty list of theorems is the same as not
chaining at all. Thus ``@{command "from"}~@{text nothing}'' has no
effect apart from entering @{text "prove(chain)"} mode, since
@{fact_ref nothing} is bound to the empty list of theorems.
Basic proof methods (such as @{method_ref rule}) expect multiple
facts to be given in their proper order, corresponding to a prefix
of the premises of the rule involved. Note that positions may be
easily skipped using something like @{command "from"}~@{text "_
\<AND> a \<AND> b"}, for example. This involves the trivial rule
@{text "PROP \<psi> \<Longrightarrow> PROP \<psi>"}, which is bound in Isabelle/Pure as
``@{fact_ref "_"}'' (underscore).
Automated methods (such as @{method simp} or @{method auto}) just
insert any given facts before their usual operation. Depending on
the kind of procedure involved, the order of facts is less
significant here.
*}
subsection {* Goals \label{sec:goals} *}
text {*
\begin{matharray}{rcl}
@{command_def "lemma"} & : & @{text "local_theory \<rightarrow> proof(prove)"} \\
@{command_def "theorem"} & : & @{text "local_theory \<rightarrow> proof(prove)"} \\
@{command_def "corollary"} & : & @{text "local_theory \<rightarrow> proof(prove)"} \\
@{command_def "have"} & : & @{text "proof(state) | proof(chain) \<rightarrow> proof(prove)"} \\
@{command_def "show"} & : & @{text "proof(state) | proof(chain) \<rightarrow> proof(prove)"} \\
@{command_def "hence"} & : & @{text "proof(state) \<rightarrow> proof(prove)"} \\
@{command_def "thus"} & : & @{text "proof(state) \<rightarrow> proof(prove)"} \\
@{command_def "print_statement"}@{text "\<^sup>*"} & : & @{text "context \<rightarrow>"} \\
\end{matharray}
From a theory context, proof mode is entered by an initial goal
command such as @{command "lemma"}, @{command "theorem"}, or
@{command "corollary"}. Within a proof, new claims may be
introduced locally as well; four variants are available here to
indicate whether forward chaining of facts should be performed
initially (via @{command_ref "then"}), and whether the final result
is meant to solve some pending goal.
Goals may consist of multiple statements, resulting in a list of
facts eventually. A pending multi-goal is internally represented as
a meta-level conjunction (@{text "&&&"}), which is usually
split into the corresponding number of sub-goals prior to an initial
method application, via @{command_ref "proof"}
(\secref{sec:proof-steps}) or @{command_ref "apply"}
(\secref{sec:tactic-commands}). The @{method_ref induct} method
covered in \secref{sec:cases-induct} acts on multiple claims
simultaneously.
Claims at the theory level may be either in short or long form. A
short goal merely consists of several simultaneous propositions
(often just one). A long goal includes an explicit context
specification for the subsequent conclusion, involving local
parameters and assumptions. Here the role of each part of the
statement is explicitly marked by separate keywords (see also
\secref{sec:locale}); the local assumptions being introduced here
are available as @{fact_ref assms} in the proof. Moreover, there
are two kinds of conclusions: @{element_def "shows"} states several
simultaneous propositions (essentially a big conjunction), while
@{element_def "obtains"} claims several simultaneous simultaneous
contexts of (essentially a big disjunction of eliminated parameters
and assumptions, cf.\ \secref{sec:obtain}).
\begin{rail}
('lemma' | 'theorem' | 'corollary') target? (goal | longgoal)
;
('have' | 'show' | 'hence' | 'thus') goal
;
'print\_statement' modes? thmrefs
;
goal: (props + 'and')
;
longgoal: thmdecl? (contextelem *) conclusion
;
conclusion: 'shows' goal | 'obtains' (parname? case + '|')
;
case: (vars + 'and') 'where' (props + 'and')
;
\end{rail}
\begin{description}
\item @{command "lemma"}~@{text "a: \<phi>"} enters proof mode with
@{text \<phi>} as main goal, eventually resulting in some fact @{text "\<turnstile>
\<phi>"} to be put back into the target context. An additional
\railnonterm{context} specification may build up an initial proof
context for the subsequent claim; this includes local definitions
and syntax as well, see the definition of @{syntax contextelem} in
\secref{sec:locale}.
\item @{command "theorem"}~@{text "a: \<phi>"} and @{command
"corollary"}~@{text "a: \<phi>"} are essentially the same as @{command
"lemma"}~@{text "a: \<phi>"}, but the facts are internally marked as
being of a different kind. This discrimination acts like a formal
comment.
\item @{command "have"}~@{text "a: \<phi>"} claims a local goal,
eventually resulting in a fact within the current logical context.
This operation is completely independent of any pending sub-goals of
an enclosing goal statements, so @{command "have"} may be freely
used for experimental exploration of potential results within a
proof body.
\item @{command "show"}~@{text "a: \<phi>"} is like @{command
"have"}~@{text "a: \<phi>"} plus a second stage to refine some pending
sub-goal for each one of the finished result, after having been
exported into the corresponding context (at the head of the
sub-proof of this @{command "show"} command).
To accommodate interactive debugging, resulting rules are printed
before being applied internally. Even more, interactive execution
of @{command "show"} predicts potential failure and displays the
resulting error as a warning beforehand. Watch out for the
following message:
%FIXME proper antiquitation
\begin{ttbox}
Problem! Local statement will fail to solve any pending goal
\end{ttbox}
\item @{command "hence"} abbreviates ``@{command "then"}~@{command
"have"}'', i.e.\ claims a local goal to be proven by forward
chaining the current facts. Note that @{command "hence"} is also
equivalent to ``@{command "from"}~@{text this}~@{command "have"}''.
\item @{command "thus"} abbreviates ``@{command "then"}~@{command
"show"}''. Note that @{command "thus"} is also equivalent to
``@{command "from"}~@{text this}~@{command "show"}''.
\item @{command "print_statement"}~@{text a} prints facts from the
current theory or proof context in long statement form, according to
the syntax for @{command "lemma"} given above.
\end{description}
Any goal statement causes some term abbreviations (such as
@{variable_ref "?thesis"}) to be bound automatically, see also
\secref{sec:term-abbrev}.
The optional case names of @{element_ref "obtains"} have a twofold
meaning: (1) during the of this claim they refer to the the local
context introductions, (2) the resulting rule is annotated
accordingly to support symbolic case splits when used with the
@{method_ref cases} method (cf.\ \secref{sec:cases-induct}).
\medskip
\begin{warn}
Isabelle/Isar suffers theory-level goal statements to contain
\emph{unbound schematic variables}, although this does not conform
to the aim of human-readable proof documents! The main problem
with schematic goals is that the actual outcome is usually hard to
predict, depending on the behavior of the proof methods applied
during the course of reasoning. Note that most semi-automated
methods heavily depend on several kinds of implicit rule
declarations within the current theory context. As this would
also result in non-compositional checking of sub-proofs,
\emph{local goals} are not allowed to be schematic at all.
Nevertheless, schematic goals do have their use in Prolog-style
interactive synthesis of proven results, usually by stepwise
refinement via emulation of traditional Isabelle tactic scripts
(see also \secref{sec:tactic-commands}). In any case, users
should know what they are doing.
\end{warn}
*}
section {* Refinement steps *}
subsection {* Proof method expressions \label{sec:proof-meth} *}
text {*
Proof methods are either basic ones, or expressions composed of
methods via ``@{verbatim ","}'' (sequential composition),
``@{verbatim "|"}'' (alternative choices), ``@{verbatim "?"}''
(try), ``@{verbatim "+"}'' (repeat at least once), ``@{verbatim
"["}@{text n}@{verbatim "]"}'' (restriction to first @{text n}
sub-goals, with default @{text "n = 1"}). In practice, proof
methods are usually just a comma separated list of
\railqtok{nameref}~\railnonterm{args} specifications. Note that
parentheses may be dropped for single method specifications (with no
arguments).
\indexouternonterm{method}
\begin{rail}
method: (nameref | '(' methods ')') (() | '?' | '+' | '[' nat? ']')
;
methods: (nameref args | method) + (',' | '|')
;
\end{rail}
Proper Isar proof methods do \emph{not} admit arbitrary goal
addressing, but refer either to the first sub-goal or all sub-goals
uniformly. The goal restriction operator ``@{text "[n]"}''
evaluates a method expression within a sandbox consisting of the
first @{text n} sub-goals (which need to exist). For example, the
method ``@{text "simp_all[3]"}'' simplifies the first three
sub-goals, while ``@{text "(rule foo, simp_all)[]"}'' simplifies all
new goals that emerge from applying rule @{text "foo"} to the
originally first one.
Improper methods, notably tactic emulations, offer a separate
low-level goal addressing scheme as explicit argument to the
individual tactic being involved. Here ``@{text "[!]"}'' refers to
all goals, and ``@{text "[n-]"}'' to all goals starting from @{text
"n"}.
\indexouternonterm{goalspec}
\begin{rail}
goalspec: '[' (nat '-' nat | nat '-' | nat | '!' ) ']'
;
\end{rail}
*}
subsection {* Initial and terminal proof steps \label{sec:proof-steps} *}
text {*
\begin{matharray}{rcl}
@{command_def "proof"} & : & @{text "proof(prove) \<rightarrow> proof(state)"} \\
@{command_def "qed"} & : & @{text "proof(state) \<rightarrow> proof(state) | local_theory | theory"} \\
@{command_def "by"} & : & @{text "proof(prove) \<rightarrow> proof(state) | local_theory | theory"} \\
@{command_def ".."} & : & @{text "proof(prove) \<rightarrow> proof(state) | local_theory | theory"} \\
@{command_def "."} & : & @{text "proof(prove) \<rightarrow> proof(state) | local_theory | theory"} \\
@{command_def "sorry"} & : & @{text "proof(prove) \<rightarrow> proof(state) | local_theory | theory"} \\
\end{matharray}
Arbitrary goal refinement via tactics is considered harmful.
Structured proof composition in Isar admits proof methods to be
invoked in two places only.
\begin{enumerate}
\item An \emph{initial} refinement step @{command_ref
"proof"}~@{text "m\<^sub>1"} reduces a newly stated goal to a number
of sub-goals that are to be solved later. Facts are passed to
@{text "m\<^sub>1"} for forward chaining, if so indicated by @{text
"proof(chain)"} mode.
\item A \emph{terminal} conclusion step @{command_ref "qed"}~@{text
"m\<^sub>2"} is intended to solve remaining goals. No facts are
passed to @{text "m\<^sub>2"}.
\end{enumerate}
The only other (proper) way to affect pending goals in a proof body
is by @{command_ref "show"}, which involves an explicit statement of
what is to be solved eventually. Thus we avoid the fundamental
problem of unstructured tactic scripts that consist of numerous
consecutive goal transformations, with invisible effects.
\medskip As a general rule of thumb for good proof style, initial
proof methods should either solve the goal completely, or constitute
some well-understood reduction to new sub-goals. Arbitrary
automatic proof tools that are prone leave a large number of badly
structured sub-goals are no help in continuing the proof document in
an intelligible manner.
Unless given explicitly by the user, the default initial method is
``@{method_ref rule}'', which applies a single standard elimination
or introduction rule according to the topmost symbol involved.
There is no separate default terminal method. Any remaining goals
are always solved by assumption in the very last step.
\begin{rail}
'proof' method?
;
'qed' method?
;
'by' method method?
;
('.' | '..' | 'sorry')
;
\end{rail}
\begin{description}
\item @{command "proof"}~@{text "m\<^sub>1"} refines the goal by proof
method @{text "m\<^sub>1"}; facts for forward chaining are passed if so
indicated by @{text "proof(chain)"} mode.
\item @{command "qed"}~@{text "m\<^sub>2"} refines any remaining goals by
proof method @{text "m\<^sub>2"} and concludes the sub-proof by assumption.
If the goal had been @{text "show"} (or @{text "thus"}), some
pending sub-goal is solved as well by the rule resulting from the
result \emph{exported} into the enclosing goal context. Thus @{text
"qed"} may fail for two reasons: either @{text "m\<^sub>2"} fails, or the
resulting rule does not fit to any pending goal\footnote{This
includes any additional ``strong'' assumptions as introduced by
@{command "assume"}.} of the enclosing context. Debugging such a
situation might involve temporarily changing @{command "show"} into
@{command "have"}, or weakening the local context by replacing
occurrences of @{command "assume"} by @{command "presume"}.
\item @{command "by"}~@{text "m\<^sub>1 m\<^sub>2"} is a \emph{terminal
proof}\index{proof!terminal}; it abbreviates @{command
"proof"}~@{text "m\<^sub>1"}~@{text "qed"}~@{text "m\<^sub>2"}, but with
backtracking across both methods. Debugging an unsuccessful
@{command "by"}~@{text "m\<^sub>1 m\<^sub>2"} command can be done by expanding its
definition; in many cases @{command "proof"}~@{text "m\<^sub>1"} (or even
@{text "apply"}~@{text "m\<^sub>1"}) is already sufficient to see the
problem.
\item ``@{command ".."}'' is a \emph{default
proof}\index{proof!default}; it abbreviates @{command "by"}~@{text
"rule"}.
\item ``@{command "."}'' is a \emph{trivial
proof}\index{proof!trivial}; it abbreviates @{command "by"}~@{text
"this"}.
\item @{command "sorry"} is a \emph{fake proof}\index{proof!fake}
pretending to solve the pending claim without further ado. This
only works in interactive development, or if the @{ML
quick_and_dirty} flag is enabled (in ML). Facts emerging from fake
proofs are not the real thing. Internally, each theorem container
is tainted by an oracle invocation, which is indicated as ``@{text
"[!]"}'' in the printed result.
The most important application of @{command "sorry"} is to support
experimentation and top-down proof development.
\end{description}
*}
subsection {* Fundamental methods and attributes \label{sec:pure-meth-att} *}
text {*
The following proof methods and attributes refer to basic logical
operations of Isar. Further methods and attributes are provided by
several generic and object-logic specific tools and packages (see
\chref{ch:gen-tools} and \chref{ch:hol}).
\begin{matharray}{rcl}
@{method_def "-"} & : & @{text method} \\
@{method_def "fact"} & : & @{text method} \\
@{method_def "assumption"} & : & @{text method} \\
@{method_def "this"} & : & @{text method} \\
@{method_def "rule"} & : & @{text method} \\
@{attribute_def (Pure) "intro"} & : & @{text attribute} \\
@{attribute_def (Pure) "elim"} & : & @{text attribute} \\
@{attribute_def (Pure) "dest"} & : & @{text attribute} \\
@{attribute_def "rule"} & : & @{text attribute} \\[0.5ex]
@{attribute_def "OF"} & : & @{text attribute} \\
@{attribute_def "of"} & : & @{text attribute} \\
@{attribute_def "where"} & : & @{text attribute} \\
\end{matharray}
\begin{rail}
'fact' thmrefs?
;
'rule' thmrefs?
;
rulemod: ('intro' | 'elim' | 'dest') ((('!' | () | '?') nat?) | 'del') ':' thmrefs
;
('intro' | 'elim' | 'dest') ('!' | () | '?') nat?
;
'rule' 'del'
;
'OF' thmrefs
;
'of' insts ('concl' ':' insts)?
;
'where' ((name | var | typefree | typevar) '=' (type | term) * 'and')
;
\end{rail}
\begin{description}
\item ``@{method "-"}'' (minus) does nothing but insert the forward
chaining facts as premises into the goal. Note that command
@{command_ref "proof"} without any method actually performs a single
reduction step using the @{method_ref rule} method; thus a plain
\emph{do-nothing} proof step would be ``@{command "proof"}~@{text
"-"}'' rather than @{command "proof"} alone.
\item @{method "fact"}~@{text "a\<^sub>1 \<dots> a\<^sub>n"} composes some fact from
@{text "a\<^sub>1, \<dots>, a\<^sub>n"} (or implicitly from the current proof context)
modulo unification of schematic type and term variables. The rule
structure is not taken into account, i.e.\ meta-level implication is
considered atomic. This is the same principle underlying literal
facts (cf.\ \secref{sec:syn-att}): ``@{command "have"}~@{text
"\<phi>"}~@{command "by"}~@{text fact}'' is equivalent to ``@{command
"note"}~@{verbatim "`"}@{text \<phi>}@{verbatim "`"}'' provided that
@{text "\<turnstile> \<phi>"} is an instance of some known @{text "\<turnstile> \<phi>"} in the
proof context.
\item @{method assumption} solves some goal by a single assumption
step. All given facts are guaranteed to participate in the
refinement; this means there may be only 0 or 1 in the first place.
Recall that @{command "qed"} (\secref{sec:proof-steps}) already
concludes any remaining sub-goals by assumption, so structured
proofs usually need not quote the @{method assumption} method at
all.
\item @{method this} applies all of the current facts directly as
rules. Recall that ``@{command "."}'' (dot) abbreviates ``@{command
"by"}~@{text this}''.
\item @{method rule}~@{text "a\<^sub>1 \<dots> a\<^sub>n"} applies some rule given as
argument in backward manner; facts are used to reduce the rule
before applying it to the goal. Thus @{method rule} without facts
is plain introduction, while with facts it becomes elimination.
When no arguments are given, the @{method rule} method tries to pick
appropriate rules automatically, as declared in the current context
using the @{attribute (Pure) intro}, @{attribute (Pure) elim},
@{attribute (Pure) dest} attributes (see below). This is the
default behavior of @{command "proof"} and ``@{command ".."}''
(double-dot) steps (see \secref{sec:proof-steps}).
\item @{attribute (Pure) intro}, @{attribute (Pure) elim}, and
@{attribute (Pure) dest} declare introduction, elimination, and
destruct rules, to be used with method @{method rule}, and similar
tools. Note that the latter will ignore rules declared with
``@{text "?"}'', while ``@{text "!"}'' are used most aggressively.
The classical reasoner (see \secref{sec:classical}) introduces its
own variants of these attributes; use qualified names to access the
present versions of Isabelle/Pure, i.e.\ @{attribute (Pure)
"Pure.intro"}.
\item @{attribute rule}~@{text del} undeclares introduction,
elimination, or destruct rules.
\item @{attribute OF}~@{text "a\<^sub>1 \<dots> a\<^sub>n"} applies some theorem to all
of the given rules @{text "a\<^sub>1, \<dots>, a\<^sub>n"} (in parallel). This
corresponds to the @{ML [source=false] "op MRS"} operation in ML,
but note the reversed order. Positions may be effectively skipped
by including ``@{text _}'' (underscore) as argument.
\item @{attribute of}~@{text "t\<^sub>1 \<dots> t\<^sub>n"} performs positional
instantiation of term variables. The terms @{text "t\<^sub>1, \<dots>, t\<^sub>n"} are
substituted for any schematic variables occurring in a theorem from
left to right; ``@{text _}'' (underscore) indicates to skip a
position. Arguments following a ``@{text "concl:"}'' specification
refer to positions of the conclusion of a rule.
\item @{attribute "where"}~@{text "x\<^sub>1 = t\<^sub>1 \<AND> \<dots> x\<^sub>n = t\<^sub>n"}
performs named instantiation of schematic type and term variables
occurring in a theorem. Schematic variables have to be specified on
the left-hand side (e.g.\ @{text "?x1.3"}). The question mark may
be omitted if the variable name is a plain identifier without index.
As type instantiations are inferred from term instantiations,
explicit type instantiations are seldom necessary.
\end{description}
*}
subsection {* Emulating tactic scripts \label{sec:tactic-commands} *}
text {*
The Isar provides separate commands to accommodate tactic-style
proof scripts within the same system. While being outside the
orthodox Isar proof language, these might come in handy for
interactive exploration and debugging, or even actual tactical proof
within new-style theories (to benefit from document preparation, for
example). See also \secref{sec:tactics} for actual tactics, that
have been encapsulated as proof methods. Proper proof methods may
be used in scripts, too.
\begin{matharray}{rcl}
@{command_def "apply"}@{text "\<^sup>*"} & : & @{text "proof(prove) \<rightarrow> proof(prove)"} \\
@{command_def "apply_end"}@{text "\<^sup>*"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "done"}@{text "\<^sup>*"} & : & @{text "proof(prove) \<rightarrow> proof(state) | local_theory | theory"} \\
@{command_def "defer"}@{text "\<^sup>*"} & : & @{text "proof \<rightarrow> proof"} \\
@{command_def "prefer"}@{text "\<^sup>*"} & : & @{text "proof \<rightarrow> proof"} \\
@{command_def "back"}@{text "\<^sup>*"} & : & @{text "proof \<rightarrow> proof"} \\
\end{matharray}
\begin{rail}
( 'apply' | 'apply\_end' ) method
;
'defer' nat?
;
'prefer' nat
;
\end{rail}
\begin{description}
\item @{command "apply"}~@{text m} applies proof method @{text m} in
initial position, but unlike @{command "proof"} it retains ``@{text
"proof(prove)"}'' mode. Thus consecutive method applications may be
given just as in tactic scripts.
Facts are passed to @{text m} as indicated by the goal's
forward-chain mode, and are \emph{consumed} afterwards. Thus any
further @{command "apply"} command would always work in a purely
backward manner.
\item @{command "apply_end"}~@{text "m"} applies proof method @{text
m} as if in terminal position. Basically, this simulates a
multi-step tactic script for @{command "qed"}, but may be given
anywhere within the proof body.
No facts are passed to @{text m} here. Furthermore, the static
context is that of the enclosing goal (as for actual @{command
"qed"}). Thus the proof method may not refer to any assumptions
introduced in the current body, for example.
\item @{command "done"} completes a proof script, provided that the
current goal state is solved completely. Note that actual
structured proof commands (e.g.\ ``@{command "."}'' or @{command
"sorry"}) may be used to conclude proof scripts as well.
\item @{command "defer"}~@{text n} and @{command "prefer"}~@{text n}
shuffle the list of pending goals: @{command "defer"} puts off
sub-goal @{text n} to the end of the list (@{text "n = 1"} by
default), while @{command "prefer"} brings sub-goal @{text n} to the
front.
\item @{command "back"} does back-tracking over the result sequence
of the latest proof command. Basically, any proof command may
return multiple results.
\end{description}
Any proper Isar proof method may be used with tactic script commands
such as @{command "apply"}. A few additional emulations of actual
tactics are provided as well; these would be never used in actual
structured proofs, of course.
*}
subsection {* Defining proof methods *}
text {*
\begin{matharray}{rcl}
@{command_def "method_setup"} & : & @{text "theory \<rightarrow> theory"} \\
\end{matharray}
\begin{rail}
'method\_setup' name '=' text text
;
\end{rail}
\begin{description}
\item @{command "method_setup"}~@{text "name = text description"}
defines a proof method in the current theory. The given @{text
"text"} has to be an ML expression of type @{ML_type [source=false]
"Args.src -> Proof.context -> Proof.method"}. Parsing concrete
method syntax from @{ML_type Args.src} input can be quite tedious in
general. The following simple examples are for methods without any
explicit arguments, or a list of theorems, respectively.
%FIXME proper antiquotations
{\footnotesize
\begin{verbatim}
Method.no_args (Method.METHOD (fn facts => foobar_tac))
Method.thms_args (fn thms => Method.METHOD (fn facts => foobar_tac))
Method.ctxt_args (fn ctxt => Method.METHOD (fn facts => foobar_tac))
Method.thms_ctxt_args (fn thms => fn ctxt =>
Method.METHOD (fn facts => foobar_tac))
\end{verbatim}
}
Note that mere tactic emulations may ignore the @{text facts}
parameter above. Proper proof methods would do something
appropriate with the list of current facts, though. Single-rule
methods usually do strict forward-chaining (e.g.\ by using @{ML
Drule.multi_resolves}), while automatic ones just insert the facts
using @{ML Method.insert_tac} before applying the main tactic.
\end{description}
*}
section {* Generalized elimination \label{sec:obtain} *}
text {*
\begin{matharray}{rcl}
@{command_def "obtain"} & : & @{text "proof(state) | proof(chain) \<rightarrow> proof(prove)"} \\
@{command_def "guess"}@{text "\<^sup>*"} & : & @{text "proof(state) | proof(chain) \<rightarrow> proof(prove)"} \\
\end{matharray}
Generalized elimination means that additional elements with certain
properties may be introduced in the current context, by virtue of a
locally proven ``soundness statement''. Technically speaking, the
@{command "obtain"} language element is like a declaration of
@{command "fix"} and @{command "assume"} (see also see
\secref{sec:proof-context}), together with a soundness proof of its
additional claim. According to the nature of existential reasoning,
assumptions get eliminated from any result exported from the context
later, provided that the corresponding parameters do \emph{not}
occur in the conclusion.
\begin{rail}
'obtain' parname? (vars + 'and') 'where' (props + 'and')
;
'guess' (vars + 'and')
;
\end{rail}
The derived Isar command @{command "obtain"} is defined as follows
(where @{text "b\<^sub>1, \<dots>, b\<^sub>k"} shall refer to (optional)
facts indicated for forward chaining).
\begin{matharray}{l}
@{text "\<langle>using b\<^sub>1 \<dots> b\<^sub>k\<rangle>"}~~@{command "obtain"}~@{text "x\<^sub>1 \<dots> x\<^sub>m \<WHERE> a: \<phi>\<^sub>1 \<dots> \<phi>\<^sub>n \<langle>proof\<rangle> \<equiv>"} \\[1ex]
\quad @{command "have"}~@{text "\<And>thesis. (\<And>x\<^sub>1 \<dots> x\<^sub>m. \<phi>\<^sub>1 \<Longrightarrow> \<dots> \<phi>\<^sub>n \<Longrightarrow> thesis) \<Longrightarrow> thesis"} \\
\quad @{command "proof"}~@{method succeed} \\
\qquad @{command "fix"}~@{text thesis} \\
\qquad @{command "assume"}~@{text "that [Pure.intro?]: \<And>x\<^sub>1 \<dots> x\<^sub>m. \<phi>\<^sub>1 \<Longrightarrow> \<dots> \<phi>\<^sub>n \<Longrightarrow> thesis"} \\
\qquad @{command "then"}~@{command "show"}~@{text thesis} \\
\quad\qquad @{command "apply"}~@{text -} \\
\quad\qquad @{command "using"}~@{text "b\<^sub>1 \<dots> b\<^sub>k \<langle>proof\<rangle>"} \\
\quad @{command "qed"} \\
\quad @{command "fix"}~@{text "x\<^sub>1 \<dots> x\<^sub>m"}~@{command "assume"}@{text "\<^sup>* a: \<phi>\<^sub>1 \<dots> \<phi>\<^sub>n"} \\
\end{matharray}
Typically, the soundness proof is relatively straight-forward, often
just by canonical automated tools such as ``@{command "by"}~@{text
simp}'' or ``@{command "by"}~@{text blast}''. Accordingly, the
``@{text that}'' reduction above is declared as simplification and
introduction rule.
In a sense, @{command "obtain"} represents at the level of Isar
proofs what would be meta-logical existential quantifiers and
conjunctions. This concept has a broad range of useful
applications, ranging from plain elimination (or introduction) of
object-level existential and conjunctions, to elimination over
results of symbolic evaluation of recursive definitions, for
example. Also note that @{command "obtain"} without parameters acts
much like @{command "have"}, where the result is treated as a
genuine assumption.
An alternative name to be used instead of ``@{text that}'' above may
be given in parentheses.
\medskip The improper variant @{command "guess"} is similar to
@{command "obtain"}, but derives the obtained statement from the
course of reasoning! The proof starts with a fixed goal @{text
thesis}. The subsequent proof may refine this to anything of the
form like @{text "\<And>x\<^sub>1 \<dots> x\<^sub>m. \<phi>\<^sub>1 \<Longrightarrow> \<dots>
\<phi>\<^sub>n \<Longrightarrow> thesis"}, but must not introduce new subgoals. The
final goal state is then used as reduction rule for the obtain
scheme described above. Obtained parameters @{text "x\<^sub>1, \<dots>,
x\<^sub>m"} are marked as internal by default, which prevents the
proof context from being polluted by ad-hoc variables. The variable
names and type constraints given as arguments for @{command "guess"}
specify a prefix of obtained parameters explicitly in the text.
It is important to note that the facts introduced by @{command
"obtain"} and @{command "guess"} may not be polymorphic: any
type-variables occurring here are fixed in the present context!
*}
section {* Calculational reasoning \label{sec:calculation} *}
text {*
\begin{matharray}{rcl}
@{command_def "also"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "finally"} & : & @{text "proof(state) \<rightarrow> proof(chain)"} \\
@{command_def "moreover"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "ultimately"} & : & @{text "proof(state) \<rightarrow> proof(chain)"} \\
@{command_def "print_trans_rules"}@{text "\<^sup>*"} & : & @{text "context \<rightarrow>"} \\
@{attribute trans} & : & @{text attribute} \\
@{attribute sym} & : & @{text attribute} \\
@{attribute symmetric} & : & @{text attribute} \\
\end{matharray}
Calculational proof is forward reasoning with implicit application
of transitivity rules (such those of @{text "="}, @{text "\<le>"},
@{text "<"}). Isabelle/Isar maintains an auxiliary fact register
@{fact_ref calculation} for accumulating results obtained by
transitivity composed with the current result. Command @{command
"also"} updates @{fact calculation} involving @{fact this}, while
@{command "finally"} exhibits the final @{fact calculation} by
forward chaining towards the next goal statement. Both commands
require valid current facts, i.e.\ may occur only after commands
that produce theorems such as @{command "assume"}, @{command
"note"}, or some finished proof of @{command "have"}, @{command
"show"} etc. The @{command "moreover"} and @{command "ultimately"}
commands are similar to @{command "also"} and @{command "finally"},
but only collect further results in @{fact calculation} without
applying any rules yet.
Also note that the implicit term abbreviation ``@{text "\<dots>"}'' has
its canonical application with calculational proofs. It refers to
the argument of the preceding statement. (The argument of a curried
infix expression happens to be its right-hand side.)
Isabelle/Isar calculations are implicitly subject to block structure
in the sense that new threads of calculational reasoning are
commenced for any new block (as opened by a local goal, for
example). This means that, apart from being able to nest
calculations, there is no separate \emph{begin-calculation} command
required.
\medskip The Isar calculation proof commands may be defined as
follows:\footnote{We suppress internal bookkeeping such as proper
handling of block-structure.}
\begin{matharray}{rcl}
@{command "also"}@{text "\<^sub>0"} & \equiv & @{command "note"}~@{text "calculation = this"} \\
@{command "also"}@{text "\<^sub>n\<^sub>+\<^sub>1"} & \equiv & @{command "note"}~@{text "calculation = trans [OF calculation this]"} \\[0.5ex]
@{command "finally"} & \equiv & @{command "also"}~@{command "from"}~@{text calculation} \\[0.5ex]
@{command "moreover"} & \equiv & @{command "note"}~@{text "calculation = calculation this"} \\
@{command "ultimately"} & \equiv & @{command "moreover"}~@{command "from"}~@{text calculation} \\
\end{matharray}
\begin{rail}
('also' | 'finally') ('(' thmrefs ')')?
;
'trans' (() | 'add' | 'del')
;
\end{rail}
\begin{description}
\item @{command "also"}~@{text "(a\<^sub>1 \<dots> a\<^sub>n)"} maintains the auxiliary
@{fact calculation} register as follows. The first occurrence of
@{command "also"} in some calculational thread initializes @{fact
calculation} by @{fact this}. Any subsequent @{command "also"} on
the same level of block-structure updates @{fact calculation} by
some transitivity rule applied to @{fact calculation} and @{fact
this} (in that order). Transitivity rules are picked from the
current context, unless alternative rules are given as explicit
arguments.
\item @{command "finally"}~@{text "(a\<^sub>1 \<dots> a\<^sub>n)"} maintaining @{fact
calculation} in the same way as @{command "also"}, and concludes the
current calculational thread. The final result is exhibited as fact
for forward chaining towards the next goal. Basically, @{command
"finally"} just abbreviates @{command "also"}~@{command
"from"}~@{fact calculation}. Typical idioms for concluding
calculational proofs are ``@{command "finally"}~@{command
"show"}~@{text ?thesis}~@{command "."}'' and ``@{command
"finally"}~@{command "have"}~@{text \<phi>}~@{command "."}''.
\item @{command "moreover"} and @{command "ultimately"} are
analogous to @{command "also"} and @{command "finally"}, but collect
results only, without applying rules.
\item @{command "print_trans_rules"} prints the list of transitivity
rules (for calculational commands @{command "also"} and @{command
"finally"}) and symmetry rules (for the @{attribute symmetric}
operation and single step elimination patters) of the current
context.
\item @{attribute trans} declares theorems as transitivity rules.
\item @{attribute sym} declares symmetry rules, as well as
@{attribute "Pure.elim"}@{text "?"} rules.
\item @{attribute symmetric} resolves a theorem with some rule
declared as @{attribute sym} in the current context. For example,
``@{command "assume"}~@{text "[symmetric]: x = y"}'' produces a
swapped fact derived from that assumption.
In structured proof texts it is often more appropriate to use an
explicit single-step elimination proof, such as ``@{command
"assume"}~@{text "x = y"}~@{command "then"}~@{command "have"}~@{text
"y = x"}~@{command ".."}''.
\end{description}
*}
section {* Proof by cases and induction \label{sec:cases-induct} *}
subsection {* Rule contexts *}
text {*
\begin{matharray}{rcl}
@{command_def "case"} & : & @{text "proof(state) \<rightarrow> proof(state)"} \\
@{command_def "print_cases"}@{text "\<^sup>*"} & : & @{text "context \<rightarrow>"} \\
@{attribute_def case_names} & : & @{text attribute} \\
@{attribute_def case_conclusion} & : & @{text attribute} \\
@{attribute_def params} & : & @{text attribute} \\
@{attribute_def consumes} & : & @{text attribute} \\
\end{matharray}
The puristic way to build up Isar proof contexts is by explicit
language elements like @{command "fix"}, @{command "assume"},
@{command "let"} (see \secref{sec:proof-context}). This is adequate
for plain natural deduction, but easily becomes unwieldy in concrete
verification tasks, which typically involve big induction rules with
several cases.
The @{command "case"} command provides a shorthand to refer to a
local context symbolically: certain proof methods provide an
environment of named ``cases'' of the form @{text "c: x\<^sub>1, \<dots>,
x\<^sub>m, \<phi>\<^sub>1, \<dots>, \<phi>\<^sub>n"}; the effect of ``@{command
"case"}~@{text c}'' is then equivalent to ``@{command "fix"}~@{text
"x\<^sub>1 \<dots> x\<^sub>m"}~@{command "assume"}~@{text "c: \<phi>\<^sub>1 \<dots>
\<phi>\<^sub>n"}''. Term bindings may be covered as well, notably
@{variable ?case} for the main conclusion.
By default, the ``terminology'' @{text "x\<^sub>1, \<dots>, x\<^sub>m"} of
a case value is marked as hidden, i.e.\ there is no way to refer to
such parameters in the subsequent proof text. After all, original
rule parameters stem from somewhere outside of the current proof
text. By using the explicit form ``@{command "case"}~@{text "(c
y\<^sub>1 \<dots> y\<^sub>m)"}'' instead, the proof author is able to
chose local names that fit nicely into the current context.
\medskip It is important to note that proper use of @{command
"case"} does not provide means to peek at the current goal state,
which is not directly observable in Isar! Nonetheless, goal
refinement commands do provide named cases @{text "goal\<^sub>i"}
for each subgoal @{text "i = 1, \<dots>, n"} of the resulting goal state.
Using this extra feature requires great care, because some bits of
the internal tactical machinery intrude the proof text. In
particular, parameter names stemming from the left-over of automated
reasoning tools are usually quite unpredictable.
Under normal circumstances, the text of cases emerge from standard
elimination or induction rules, which in turn are derived from
previous theory specifications in a canonical way (say from
@{command "inductive"} definitions).
\medskip Proper cases are only available if both the proof method
and the rules involved support this. By using appropriate
attributes, case names, conclusions, and parameters may be also
declared by hand. Thus variant versions of rules that have been
derived manually become ready to use in advanced case analysis
later.
\begin{rail}
'case' (caseref | '(' caseref ((name | underscore) +) ')')
;
caseref: nameref attributes?
;
'case\_names' (name +)
;
'case\_conclusion' name (name *)
;
'params' ((name *) + 'and')
;
'consumes' nat?
;
\end{rail}
\begin{description}
\item @{command "case"}~@{text "(c x\<^sub>1 \<dots> x\<^sub>m)"} invokes a named local
context @{text "c: x\<^sub>1, \<dots>, x\<^sub>m, \<phi>\<^sub>1, \<dots>, \<phi>\<^sub>m"}, as provided by an
appropriate proof method (such as @{method_ref cases} and
@{method_ref induct}). The command ``@{command "case"}~@{text "(c
x\<^sub>1 \<dots> x\<^sub>m)"}'' abbreviates ``@{command "fix"}~@{text "x\<^sub>1 \<dots>
x\<^sub>m"}~@{command "assume"}~@{text "c: \<phi>\<^sub>1 \<dots> \<phi>\<^sub>n"}''.
\item @{command "print_cases"} prints all local contexts of the
current state, using Isar proof language notation.
\item @{attribute case_names}~@{text "c\<^sub>1 \<dots> c\<^sub>k"} declares names for
the local contexts of premises of a theorem; @{text "c\<^sub>1, \<dots>, c\<^sub>k"}
refers to the \emph{suffix} of the list of premises.
\item @{attribute case_conclusion}~@{text "c d\<^sub>1 \<dots> d\<^sub>k"} declares
names for the conclusions of a named premise @{text c}; here @{text
"d\<^sub>1, \<dots>, d\<^sub>k"} refers to the prefix of arguments of a logical formula
built by nesting a binary connective (e.g.\ @{text "\<or>"}).
Note that proof methods such as @{method induct} and @{method
coinduct} already provide a default name for the conclusion as a
whole. The need to name subformulas only arises with cases that
split into several sub-cases, as in common co-induction rules.
\item @{attribute params}~@{text "p\<^sub>1 \<dots> p\<^sub>m \<AND> \<dots> q\<^sub>1 \<dots> q\<^sub>n"} renames
the innermost parameters of premises @{text "1, \<dots>, n"} of some
theorem. An empty list of names may be given to skip positions,
leaving the present parameters unchanged.
Note that the default usage of case rules does \emph{not} directly
expose parameters to the proof context.
\item @{attribute consumes}~@{text n} declares the number of ``major
premises'' of a rule, i.e.\ the number of facts to be consumed when
it is applied by an appropriate proof method. The default value of
@{attribute consumes} is @{text "n = 1"}, which is appropriate for
the usual kind of cases and induction rules for inductive sets (cf.\
\secref{sec:hol-inductive}). Rules without any @{attribute
consumes} declaration given are treated as if @{attribute
consumes}~@{text 0} had been specified.
Note that explicit @{attribute consumes} declarations are only
rarely needed; this is already taken care of automatically by the
higher-level @{attribute cases}, @{attribute induct}, and
@{attribute coinduct} declarations.
\end{description}
*}
subsection {* Proof methods *}
text {*
\begin{matharray}{rcl}
@{method_def cases} & : & @{text method} \\
@{method_def induct} & : & @{text method} \\
@{method_def coinduct} & : & @{text method} \\
\end{matharray}
The @{method cases}, @{method induct}, and @{method coinduct}
methods provide a uniform interface to common proof techniques over
datatypes, inductive predicates (or sets), recursive functions etc.
The corresponding rules may be specified and instantiated in a
casual manner. Furthermore, these methods provide named local
contexts that may be invoked via the @{command "case"} proof command
within the subsequent proof text. This accommodates compact proof
texts even when reasoning about large specifications.
The @{method induct} method also provides some additional
infrastructure in order to be applicable to structure statements
(either using explicit meta-level connectives, or including facts
and parameters separately). This avoids cumbersome encoding of
``strengthened'' inductive statements within the object-logic.
\begin{rail}
'cases' (insts * 'and') rule?
;
'induct' (definsts * 'and') \\ arbitrary? taking? rule?
;
'coinduct' insts taking rule?
;
rule: ('type' | 'pred' | 'set') ':' (nameref +) | 'rule' ':' (thmref +)
;
definst: name ('==' | equiv) term | inst
;
definsts: ( definst *)
;
arbitrary: 'arbitrary' ':' ((term *) 'and' +)
;
taking: 'taking' ':' insts
;
\end{rail}
\begin{description}
\item @{method cases}~@{text "insts R"} applies method @{method
rule} with an appropriate case distinction theorem, instantiated to
the subjects @{text insts}. Symbolic case names are bound according
to the rule's local contexts.
The rule is determined as follows, according to the facts and
arguments passed to the @{method cases} method:
\medskip
\begin{tabular}{llll}
facts & & arguments & rule \\\hline
& @{method cases} & & classical case split \\
& @{method cases} & @{text t} & datatype exhaustion (type of @{text t}) \\
@{text "\<turnstile> A t"} & @{method cases} & @{text "\<dots>"} & inductive predicate/set elimination (of @{text A}) \\
@{text "\<dots>"} & @{method cases} & @{text "\<dots> rule: R"} & explicit rule @{text R} \\
\end{tabular}
\medskip
Several instantiations may be given, referring to the \emph{suffix}
of premises of the case rule; within each premise, the \emph{prefix}
of variables is instantiated. In most situations, only a single
term needs to be specified; this refers to the first variable of the
last premise (it is usually the same for all cases).
\item @{method induct}~@{text "insts R"} is analogous to the
@{method cases} method, but refers to induction rules, which are
determined as follows:
\medskip
\begin{tabular}{llll}
facts & & arguments & rule \\\hline
& @{method induct} & @{text "P x"} & datatype induction (type of @{text x}) \\
@{text "\<turnstile> A x"} & @{method induct} & @{text "\<dots>"} & predicate/set induction (of @{text A}) \\
@{text "\<dots>"} & @{method induct} & @{text "\<dots> rule: R"} & explicit rule @{text R} \\
\end{tabular}
\medskip
Several instantiations may be given, each referring to some part of
a mutual inductive definition or datatype --- only related partial
induction rules may be used together, though. Any of the lists of
terms @{text "P, x, \<dots>"} refers to the \emph{suffix} of variables
present in the induction rule. This enables the writer to specify
only induction variables, or both predicates and variables, for
example.
Instantiations may be definitional: equations @{text "x \<equiv> t"}
introduce local definitions, which are inserted into the claim and
discharged after applying the induction rule. Equalities reappear
in the inductive cases, but have been transformed according to the
induction principle being involved here. In order to achieve
practically useful induction hypotheses, some variables occurring in
@{text t} need to be fixed (see below).
The optional ``@{text "arbitrary: x\<^sub>1 \<dots> x\<^sub>m"}''
specification generalizes variables @{text "x\<^sub>1, \<dots>,
x\<^sub>m"} of the original goal before applying induction. Thus
induction hypotheses may become sufficiently general to get the
proof through. Together with definitional instantiations, one may
effectively perform induction over expressions of a certain
structure.
The optional ``@{text "taking: t\<^sub>1 \<dots> t\<^sub>n"}''
specification provides additional instantiations of a prefix of
pending variables in the rule. Such schematic induction rules
rarely occur in practice, though.
\item @{method coinduct}~@{text "inst R"} is analogous to the
@{method induct} method, but refers to coinduction rules, which are
determined as follows:
\medskip
\begin{tabular}{llll}
goal & & arguments & rule \\\hline
& @{method coinduct} & @{text x} & type coinduction (type of @{text x}) \\
@{text "A x"} & @{method coinduct} & @{text "\<dots>"} & predicate/set coinduction (of @{text A}) \\
@{text "\<dots>"} & @{method coinduct} & @{text "\<dots> rule: R"} & explicit rule @{text R} \\
\end{tabular}
Coinduction is the dual of induction. Induction essentially
eliminates @{text "A x"} towards a generic result @{text "P x"},
while coinduction introduces @{text "A x"} starting with @{text "B
x"}, for a suitable ``bisimulation'' @{text B}. The cases of a
coinduct rule are typically named after the predicates or sets being
covered, while the conclusions consist of several alternatives being
named after the individual destructor patterns.
The given instantiation refers to the \emph{suffix} of variables
occurring in the rule's major premise, or conclusion if unavailable.
An additional ``@{text "taking: t\<^sub>1 \<dots> t\<^sub>n"}''
specification may be required in order to specify the bisimulation
to be used in the coinduction step.
\end{description}
Above methods produce named local contexts, as determined by the
instantiated rule as given in the text. Beyond that, the @{method
induct} and @{method coinduct} methods guess further instantiations
from the goal specification itself. Any persisting unresolved
schematic variables of the resulting rule will render the the
corresponding case invalid. The term binding @{variable ?case} for
the conclusion will be provided with each case, provided that term
is fully specified.
The @{command "print_cases"} command prints all named cases present
in the current proof state.
\medskip Despite the additional infrastructure, both @{method cases}
and @{method coinduct} merely apply a certain rule, after
instantiation, while conforming due to the usual way of monotonic
natural deduction: the context of a structured statement @{text
"\<And>x\<^sub>1 \<dots> x\<^sub>m. \<phi>\<^sub>1 \<Longrightarrow> \<dots> \<phi>\<^sub>n \<Longrightarrow> \<dots>"}
reappears unchanged after the case split.
The @{method induct} method is fundamentally different in this
respect: the meta-level structure is passed through the
``recursive'' course involved in the induction. Thus the original
statement is basically replaced by separate copies, corresponding to
the induction hypotheses and conclusion; the original goal context
is no longer available. Thus local assumptions, fixed parameters
and definitions effectively participate in the inductive rephrasing
of the original statement.
In induction proofs, local assumptions introduced by cases are split
into two different kinds: @{text hyps} stemming from the rule and
@{text prems} from the goal statement. This is reflected in the
extracted cases accordingly, so invoking ``@{command "case"}~@{text
c}'' will provide separate facts @{text c.hyps} and @{text c.prems},
as well as fact @{text c} to hold the all-inclusive list.
\medskip Facts presented to either method are consumed according to
the number of ``major premises'' of the rule involved, which is
usually 0 for plain cases and induction rules of datatypes etc.\ and
1 for rules of inductive predicates or sets and the like. The
remaining facts are inserted into the goal verbatim before the
actual @{text cases}, @{text induct}, or @{text coinduct} rule is
applied.
*}
subsection {* Declaring rules *}
text {*
\begin{matharray}{rcl}
@{command_def "print_induct_rules"}@{text "\<^sup>*"} & : & @{text "context \<rightarrow>"} \\
@{attribute_def cases} & : & @{text attribute} \\
@{attribute_def induct} & : & @{text attribute} \\
@{attribute_def coinduct} & : & @{text attribute} \\
\end{matharray}
\begin{rail}
'cases' spec
;
'induct' spec
;
'coinduct' spec
;
spec: (('type' | 'pred' | 'set') ':' nameref) | 'del'
;
\end{rail}
\begin{description}
\item @{command "print_induct_rules"} prints cases and induct rules
for predicates (or sets) and types of the current context.
\item @{attribute cases}, @{attribute induct}, and @{attribute
coinduct} (as attributes) declare rules for reasoning about
(co)inductive predicates (or sets) and types, using the
corresponding methods of the same name. Certain definitional
packages of object-logics usually declare emerging cases and
induction rules as expected, so users rarely need to intervene.
Rules may be deleted via the @{text "del"} specification, which
covers all of the @{text "type"}/@{text "pred"}/@{text "set"}
sub-categories simultaneously. For example, @{attribute
cases}~@{text del} removes any @{attribute cases} rules declared for
some type, predicate, or set.
Manual rule declarations usually refer to the @{attribute
case_names} and @{attribute params} attributes to adjust names of
cases and parameters of a rule; the @{attribute consumes}
declaration is taken care of automatically: @{attribute
consumes}~@{text 0} is specified for ``type'' rules and @{attribute
consumes}~@{text 1} for ``predicate'' / ``set'' rules.
\end{description}
*}
end