doc-src/IsarRef/Thy/Synopsis.thy

Using mathematical notation for <-> and cardinal arithmetic

theory Synopsis
imports Base Main
begin

chapter {* Synopsis *}

section {* Notepad *}

text {*
  An Isar proof body serves as mathematical notepad to compose logical
  content, consisting of types, terms, facts.
*}


subsection {* Types and terms *}

notepad
begin
  txt {* Locally fixed entities: *}
  fix x   -- {* local constant, without any type information yet *}
  fix x :: 'a  -- {* variant with explicit type-constraint for subsequent use*}

  fix a b
  assume "a = b"  -- {* type assignment at first occurrence in concrete term *}

  txt {* Definitions (non-polymorphic): *}
  def x \<equiv> "t::'a"

  txt {* Abbreviations (polymorphic): *}
  let ?f = "\<lambda>x. x"
  term "?f ?f"

  txt {* Notation: *}
  write x  ("***")
end


subsection {* Facts *}

text {*
  A fact is a simultaneous list of theorems.
*}


subsubsection {* Producing facts *}

notepad
begin

  txt {* Via assumption (``lambda''): *}
  assume a: A

  txt {* Via proof (``let''): *}
  have b: B sorry

  txt {* Via abbreviation (``let''): *}
  note c = a b

end


subsubsection {* Referencing facts *}

notepad
begin
  txt {* Via explicit name: *}
  assume a: A
  note a

  txt {* Via implicit name: *}
  assume A
  note this

  txt {* Via literal proposition (unification with results from the proof text): *}
  assume A
  note `A`

  assume "\<And>x. B x"
  note `B a`
  note `B b`
end


subsubsection {* Manipulating facts *}

notepad
begin
  txt {* Instantiation: *}
  assume a: "\<And>x. B x"
  note a
  note a [of b]
  note a [where x = b]

  txt {* Backchaining: *}
  assume 1: A
  assume 2: "A \<Longrightarrow> C"
  note 2 [OF 1]
  note 1 [THEN 2]

  txt {* Symmetric results: *}
  assume "x = y"
  note this [symmetric]

  assume "x \<noteq> y"
  note this [symmetric]

  txt {* Adhoc-simplification (take care!): *}
  assume "P ([] @ xs)"
  note this [simplified]
end


subsubsection {* Projections *}

text {*
  Isar facts consist of multiple theorems.  There is notation to project
  interval ranges.
*}

notepad
begin
  assume stuff: A B C D
  note stuff(1)
  note stuff(2-3)
  note stuff(2-)
end


subsubsection {* Naming conventions *}

text {*
  \begin{itemize}

  \item Lower-case identifiers are usually preferred.

  \item Facts can be named after the main term within the proposition.

  \item Facts should \emph{not} be named after the command that
  introduced them (@{command "assume"}, @{command "have"}).  This is
  misleading and hard to maintain.

  \item Natural numbers can be used as ``meaningless'' names (more
  appropriate than @{text "a1"}, @{text "a2"} etc.)

  \item Symbolic identifiers are supported (e.g. @{text "*"}, @{text
  "**"}, @{text "***"}).

  \end{itemize}
*}


subsection {* Block structure *}

text {*
  The formal notepad is block structured.  The fact produced by the last
  entry of a block is exported into the outer context.
*}

notepad
begin
  {
    have a: A sorry
    have b: B sorry
    note a b
  }
  note this
  note `A`
  note `B`
end

text {* Explicit blocks as well as implicit blocks of nested goal
  statements (e.g.\ @{command have}) automatically introduce one extra
  pair of parentheses in reserve.  The @{command next} command allows
  to ``jump'' between these sub-blocks. *}

notepad
begin

  {
    have a: A sorry
  next
    have b: B
    proof -
      show B sorry
    next
      have c: C sorry
    next
      have d: D sorry
    qed
  }

  txt {* Alternative version with explicit parentheses everywhere: *}

  {
    {
      have a: A sorry
    }
    {
      have b: B
      proof -
        {
          show B sorry
        }
        {
          have c: C sorry
        }
        {
          have d: D sorry
        }
      qed
    }
  }

end


section {* Calculational reasoning \label{sec:calculations-synopsis} *}

text {*
  For example, see @{file "~~/src/HOL/Isar_Examples/Group.thy"}.
*}


subsection {* Special names in Isar proofs *}

text {*
  \begin{itemize}

  \item term @{text "?thesis"} --- the main conclusion of the
  innermost pending claim

  \item term @{text "\<dots>"} --- the argument of the last explicitly
    stated result (for infix application this is the right-hand side)

  \item fact @{text "this"} --- the last result produced in the text

  \end{itemize}
*}

notepad
begin
  have "x = y"
  proof -
    term ?thesis
    show ?thesis sorry
    term ?thesis  -- {* static! *}
  qed
  term "\<dots>"
  thm this
end

text {* Calculational reasoning maintains the special fact called
  ``@{text calculation}'' in the background.  Certain language
  elements combine primary @{text this} with secondary @{text
  calculation}. *}


subsection {* Transitive chains *}

text {* The Idea is to combine @{text this} and @{text calculation}
  via typical @{text trans} rules (see also @{command
  print_trans_rules}): *}

thm trans
thm less_trans
thm less_le_trans

notepad
begin
  txt {* Plain bottom-up calculation: *}
  have "a = b" sorry
  also
  have "b = c" sorry
  also
  have "c = d" sorry
  finally
  have "a = d" .

  txt {* Variant using the @{text "\<dots>"} abbreviation: *}
  have "a = b" sorry
  also
  have "\<dots> = c" sorry
  also
  have "\<dots> = d" sorry
  finally
  have "a = d" .

  txt {* Top-down version with explicit claim at the head: *}
  have "a = d"
  proof -
    have "a = b" sorry
    also
    have "\<dots> = c" sorry
    also
    have "\<dots> = d" sorry
    finally
    show ?thesis .
  qed
next
  txt {* Mixed inequalities (require suitable base type): *}
  fix a b c d :: nat

  have "a < b" sorry
  also
  have "b \<le> c" sorry
  also
  have "c = d" sorry
  finally
  have "a < d" .
end


subsubsection {* Notes *}

text {*
  \begin{itemize}

  \item The notion of @{text trans} rule is very general due to the
  flexibility of Isabelle/Pure rule composition.

  \item User applications may declare their own rules, with some care
  about the operational details of higher-order unification.

  \end{itemize}
*}


subsection {* Degenerate calculations and bigstep reasoning *}

text {* The Idea is to append @{text this} to @{text calculation},
  without rule composition.  *}

notepad
begin
  txt {* A vacuous proof: *}
  have A sorry
  moreover
  have B sorry
  moreover
  have C sorry
  ultimately
  have A and B and C .
next
  txt {* Slightly more content (trivial bigstep reasoning): *}
  have A sorry
  moreover
  have B sorry
  moreover
  have C sorry
  ultimately
  have "A \<and> B \<and> C" by blast
next
  txt {* More ambitious bigstep reasoning involving structured results: *}
  have "A \<or> B \<or> C" sorry
  moreover
  { assume A have R sorry }
  moreover
  { assume B have R sorry }
  moreover
  { assume C have R sorry }
  ultimately
  have R by blast  -- {* ``big-bang integration'' of proof blocks (occasionally fragile) *}
end


section {* Induction *}

subsection {* Induction as Natural Deduction *}

text {* In principle, induction is just a special case of Natural
  Deduction (see also \secref{sec:natural-deduction-synopsis}).  For
  example: *}

thm nat.induct
print_statement nat.induct

notepad
begin
  fix n :: nat
  have "P n"
  proof (rule nat.induct)  -- {* fragile rule application! *}
    show "P 0" sorry
  next
    fix n :: nat
    assume "P n"
    show "P (Suc n)" sorry
  qed
end

text {*
  In practice, much more proof infrastructure is required.

  The proof method @{method induct} provides:
  \begin{itemize}

  \item implicit rule selection and robust instantiation

  \item context elements via symbolic case names

  \item support for rule-structured induction statements, with local
    parameters, premises, etc.

  \end{itemize}
*}

notepad
begin
  fix n :: nat
  have "P n"
  proof (induct n)
    case 0
    show ?case sorry
  next
    case (Suc n)
    from Suc.hyps show ?case sorry
  qed
end


subsubsection {* Example *}

text {*
  The subsequent example combines the following proof patterns:
  \begin{itemize}

  \item outermost induction (over the datatype structure of natural
  numbers), to decompose the proof problem in top-down manner

  \item calculational reasoning (\secref{sec:calculations-synopsis})
  to compose the result in each case

  \item solving local claims within the calculation by simplification

  \end{itemize}
*}

lemma
  fixes n :: nat
  shows "(\<Sum>i=0..n. i) = n * (n + 1) div 2"
proof (induct n)
  case 0
  have "(\<Sum>i=0..0. i) = (0::nat)" by simp
  also have "\<dots> = 0 * (0 + 1) div 2" by simp
  finally show ?case .
next
  case (Suc n)
  have "(\<Sum>i=0..Suc n. i) = (\<Sum>i=0..n. i) + (n + 1)" by simp
  also have "\<dots> = n * (n + 1) div 2 + (n + 1)" by (simp add: Suc.hyps)
  also have "\<dots> = (n * (n + 1) + 2 * (n + 1)) div 2" by simp
  also have "\<dots> = (Suc n * (Suc n + 1)) div 2" by simp
  finally show ?case .
qed

text {* This demonstrates how induction proofs can be done without
  having to consider the raw Natural Deduction structure. *}


subsection {* Induction with local parameters and premises *}

text {* Idea: Pure rule statements are passed through the induction
  rule.  This achieves convenient proof patterns, thanks to some
  internal trickery in the @{method induct} method.

  Important: Using compact HOL formulae with @{text "\<forall>/\<longrightarrow>"} is a
  well-known anti-pattern! It would produce useless formal noise.
*}

notepad
begin
  fix n :: nat
  fix P :: "nat \<Rightarrow> bool"
  fix Q :: "'a \<Rightarrow> nat \<Rightarrow> bool"

  have "P n"
  proof (induct n)
    case 0
    show "P 0" sorry
  next
    case (Suc n)
    from `P n` show "P (Suc n)" sorry
  qed

  have "A n \<Longrightarrow> P n"
  proof (induct n)
    case 0
    from `A 0` show "P 0" sorry
  next
    case (Suc n)
    from `A n \<Longrightarrow> P n`
      and `A (Suc n)` show "P (Suc n)" sorry
  qed

  have "\<And>x. Q x n"
  proof (induct n)
    case 0
    show "Q x 0" sorry
  next
    case (Suc n)
    from `\<And>x. Q x n` show "Q x (Suc n)" sorry
    txt {* Local quantification admits arbitrary instances: *}
    note `Q a n` and `Q b n`
  qed
end


subsection {* Implicit induction context *}

text {* The @{method induct} method can isolate local parameters and
  premises directly from the given statement.  This is convenient in
  practical applications, but requires some understanding of what is
  going on internally (as explained above).  *}

notepad
begin
  fix n :: nat
  fix Q :: "'a \<Rightarrow> nat \<Rightarrow> bool"

  fix x :: 'a
  assume "A x n"
  then have "Q x n"
  proof (induct n arbitrary: x)
    case 0
    from `A x 0` show "Q x 0" sorry
  next
    case (Suc n)
    from `\<And>x. A x n \<Longrightarrow> Q x n`  -- {* arbitrary instances can be produced here *}
      and `A x (Suc n)` show "Q x (Suc n)" sorry
  qed
end


subsection {* Advanced induction with term definitions *}

text {* Induction over subexpressions of a certain shape are delicate
  to formalize.  The Isar @{method induct} method provides
  infrastructure for this.

  Idea: sub-expressions of the problem are turned into a defined
  induction variable; often accompanied with fixing of auxiliary
  parameters in the original expression.  *}

notepad
begin
  fix a :: "'a \<Rightarrow> nat"
  fix A :: "nat \<Rightarrow> bool"

  assume "A (a x)"
  then have "P (a x)"
  proof (induct "a x" arbitrary: x)
    case 0
    note prem = `A (a x)`
      and defn = `0 = a x`
    show "P (a x)" sorry
  next
    case (Suc n)
    note hyp = `\<And>x. n = a x \<Longrightarrow> A (a x) \<Longrightarrow> P (a x)`
      and prem = `A (a x)`
      and defn = `Suc n = a x`
    show "P (a x)" sorry
  qed
end


section {* Natural Deduction \label{sec:natural-deduction-synopsis} *}

subsection {* Rule statements *}

text {*
  Isabelle/Pure ``theorems'' are always natural deduction rules,
  which sometimes happen to consist of a conclusion only.

  The framework connectives @{text "\<And>"} and @{text "\<Longrightarrow>"} indicate the
  rule structure declaratively.  For example: *}

thm conjI
thm impI
thm nat.induct

text {*
  The object-logic is embedded into the Pure framework via an implicit
  derivability judgment @{term "Trueprop :: bool \<Rightarrow> prop"}.

  Thus any HOL formulae appears atomic to the Pure framework, while
  the rule structure outlines the corresponding proof pattern.

  This can be made explicit as follows:
*}

notepad
begin
  write Trueprop  ("Tr")

  thm conjI
  thm impI
  thm nat.induct
end

text {*
  Isar provides first-class notation for rule statements as follows.
*}

print_statement conjI
print_statement impI
print_statement nat.induct


subsubsection {* Examples *}

text {*
  Introductions and eliminations of some standard connectives of
  the object-logic can be written as rule statements as follows.  (The
  proof ``@{command "by"}~@{method blast}'' serves as sanity check.)
*}

lemma "(P \<Longrightarrow> False) \<Longrightarrow> \<not> P" by blast
lemma "\<not> P \<Longrightarrow> P \<Longrightarrow> Q" by blast

lemma "P \<Longrightarrow> Q \<Longrightarrow> P \<and> Q" by blast
lemma "P \<and> Q \<Longrightarrow> (P \<Longrightarrow> Q \<Longrightarrow> R) \<Longrightarrow> R" by blast

lemma "P \<Longrightarrow> P \<or> Q" by blast
lemma "Q \<Longrightarrow> P \<or> Q" by blast
lemma "P \<or> Q \<Longrightarrow> (P \<Longrightarrow> R) \<Longrightarrow> (Q \<Longrightarrow> R) \<Longrightarrow> R" by blast

lemma "(\<And>x. P x) \<Longrightarrow> (\<forall>x. P x)" by blast
lemma "(\<forall>x. P x) \<Longrightarrow> P x" by blast

lemma "P x \<Longrightarrow> (\<exists>x. P x)" by blast
lemma "(\<exists>x. P x) \<Longrightarrow> (\<And>x. P x \<Longrightarrow> R) \<Longrightarrow> R" by blast

lemma "x \<in> A \<Longrightarrow> x \<in> B \<Longrightarrow> x \<in> A \<inter> B" by blast
lemma "x \<in> A \<inter> B \<Longrightarrow> (x \<in> A \<Longrightarrow> x \<in> B \<Longrightarrow> R) \<Longrightarrow> R" by blast

lemma "x \<in> A \<Longrightarrow> x \<in> A \<union> B" by blast
lemma "x \<in> B \<Longrightarrow> x \<in> A \<union> B" by blast
lemma "x \<in> A \<union> B \<Longrightarrow> (x \<in> A \<Longrightarrow> R) \<Longrightarrow> (x \<in> B \<Longrightarrow> R) \<Longrightarrow> R" by blast


subsection {* Isar context elements *}

text {* We derive some results out of the blue, using Isar context
  elements and some explicit blocks.  This illustrates their meaning
  wrt.\ Pure connectives, without goal states getting in the way.  *}

notepad
begin
  {
    fix x
    have "B x" sorry
  }
  have "\<And>x. B x" by fact

next

  {
    assume A
    have B sorry
  }
  have "A \<Longrightarrow> B" by fact

next

  {
    def x \<equiv> t
    have "B x" sorry
  }
  have "B t" by fact

next

  {
    obtain x :: 'a where "B x" sorry
    have C sorry
  }
  have C by fact

end


subsection {* Pure rule composition *}

text {*
  The Pure framework provides means for:

  \begin{itemize}

    \item backward-chaining of rules by @{inference resolution}

    \item closing of branches by @{inference assumption}

  \end{itemize}

  Both principles involve higher-order unification of @{text \<lambda>}-terms
  modulo @{text "\<alpha>\<beta>\<eta>"}-equivalence (cf.\ Huet and Miller).  *}

notepad
begin
  assume a: A and b: B
  thm conjI
  thm conjI [of A B]  -- "instantiation"
  thm conjI [of A B, OF a b]  -- "instantiation and composition"
  thm conjI [OF a b]  -- "composition via unification (trivial)"
  thm conjI [OF `A` `B`]

  thm conjI [OF disjI1]
end

text {* Note: Low-level rule composition is tedious and leads to
  unreadable~/ unmaintainable expressions in the text.  *}


subsection {* Structured backward reasoning *}

text {* Idea: Canonical proof decomposition via @{command fix}~/
  @{command assume}~/ @{command show}, where the body produces a
  natural deduction rule to refine some goal.  *}

notepad
begin
  fix A B :: "'a \<Rightarrow> bool"

  have "\<And>x. A x \<Longrightarrow> B x"
  proof -
    fix x
    assume "A x"
    show "B x" sorry
  qed

  have "\<And>x. A x \<Longrightarrow> B x"
  proof -
    {
      fix x
      assume "A x"
      show "B x" sorry
    } -- "implicit block structure made explicit"
    note `\<And>x. A x \<Longrightarrow> B x`
      -- "side exit for the resulting rule"
  qed
end


subsection {* Structured rule application *}

text {*
  Idea: Previous facts and new claims are composed with a rule from
  the context (or background library).
*}

notepad
begin
  assume r1: "A \<Longrightarrow> B \<Longrightarrow> C"  -- {* simple rule (Horn clause) *}

  have A sorry  -- "prefix of facts via outer sub-proof"
  then have C
  proof (rule r1)
    show B sorry  -- "remaining rule premises via inner sub-proof"
  qed

  have C
  proof (rule r1)
    show A sorry
    show B sorry
  qed

  have A and B sorry
  then have C
  proof (rule r1)
  qed

  have A and B sorry
  then have C by (rule r1)

next

  assume r2: "A \<Longrightarrow> (\<And>x. B1 x \<Longrightarrow> B2 x) \<Longrightarrow> C"  -- {* nested rule *}

  have A sorry
  then have C
  proof (rule r2)
    fix x
    assume "B1 x"
    show "B2 x" sorry
  qed

  txt {* The compound rule premise @{prop "\<And>x. B1 x \<Longrightarrow> B2 x"} is better
    addressed via @{command fix}~/ @{command assume}~/ @{command show}
    in the nested proof body.  *}
end


subsection {* Example: predicate logic *}

text {*
  Using the above principles, standard introduction and elimination proofs
  of predicate logic connectives of HOL work as follows.
*}

notepad
begin
  have "A \<longrightarrow> B" and A sorry
  then have B ..

  have A sorry
  then have "A \<or> B" ..

  have B sorry
  then have "A \<or> B" ..

  have "A \<or> B" sorry
  then have C
  proof
    assume A
    then show C sorry
  next
    assume B
    then show C sorry
  qed

  have A and B sorry
  then have "A \<and> B" ..

  have "A \<and> B" sorry
  then have A ..

  have "A \<and> B" sorry
  then have B ..

  have False sorry
  then have A ..

  have True ..

  have "\<not> A"
  proof
    assume A
    then show False sorry
  qed

  have "\<not> A" and A sorry
  then have B ..

  have "\<forall>x. P x"
  proof
    fix x
    show "P x" sorry
  qed

  have "\<forall>x. P x" sorry
  then have "P a" ..

  have "\<exists>x. P x"
  proof
    show "P a" sorry
  qed

  have "\<exists>x. P x" sorry
  then have C
  proof
    fix a
    assume "P a"
    show C sorry
  qed

  txt {* Less awkward version using @{command obtain}: *}
  have "\<exists>x. P x" sorry
  then obtain a where "P a" ..
end

text {* Further variations to illustrate Isar sub-proofs involving
  @{command show}: *}

notepad
begin
  have "A \<and> B"
  proof  -- {* two strictly isolated subproofs *}
    show A sorry
  next
    show B sorry
  qed

  have "A \<and> B"
  proof  -- {* one simultaneous sub-proof *}
    show A and B sorry
  qed

  have "A \<and> B"
  proof  -- {* two subproofs in the same context *}
    show A sorry
    show B sorry
  qed

  have "A \<and> B"
  proof  -- {* swapped order *}
    show B sorry
    show A sorry
  qed

  have "A \<and> B"
  proof  -- {* sequential subproofs *}
    show A sorry
    show B using `A` sorry
  qed
end


subsubsection {* Example: set-theoretic operators *}

text {* There is nothing special about logical connectives (@{text
  "\<and>"}, @{text "\<or>"}, @{text "\<forall>"}, @{text "\<exists>"} etc.).  Operators from
  set-theory or lattice-theory work analogously.  It is only a matter
  of rule declarations in the library; rules can be also specified
  explicitly.
*}

notepad
begin
  have "x \<in> A" and "x \<in> B" sorry
  then have "x \<in> A \<inter> B" ..

  have "x \<in> A" sorry
  then have "x \<in> A \<union> B" ..

  have "x \<in> B" sorry
  then have "x \<in> A \<union> B" ..

  have "x \<in> A \<union> B" sorry
  then have C
  proof
    assume "x \<in> A"
    then show C sorry
  next
    assume "x \<in> B"
    then show C sorry
  qed

next
  have "x \<in> \<Inter>A"
  proof
    fix a
    assume "a \<in> A"
    show "x \<in> a" sorry
  qed

  have "x \<in> \<Inter>A" sorry
  then have "x \<in> a"
  proof
    show "a \<in> A" sorry
  qed

  have "a \<in> A" and "x \<in> a" sorry
  then have "x \<in> \<Union>A" ..

  have "x \<in> \<Union>A" sorry
  then obtain a where "a \<in> A" and "x \<in> a" ..
end


section {* Generalized elimination and cases *}

subsection {* General elimination rules *}

text {*
  The general format of elimination rules is illustrated by the
  following typical representatives:
*}

thm exE     -- {* local parameter *}
thm conjE   -- {* local premises *}
thm disjE   -- {* split into cases *}

text {*
  Combining these characteristics leads to the following general scheme
  for elimination rules with cases:

  \begin{itemize}

  \item prefix of assumptions (or ``major premises'')

  \item one or more cases that enable to establish the main conclusion
    in an augmented context

  \end{itemize}
*}

notepad
begin
  assume r:
    "A1 \<Longrightarrow> A2 \<Longrightarrow>  (* assumptions *)
      (\<And>x y. B1 x y \<Longrightarrow> C1 x y \<Longrightarrow> R) \<Longrightarrow>  (* case 1 *)
      (\<And>x y. B2 x y \<Longrightarrow> C2 x y \<Longrightarrow> R) \<Longrightarrow>  (* case 2 *)
      R  (* main conclusion *)"

  have A1 and A2 sorry
  then have R
  proof (rule r)
    fix x y
    assume "B1 x y" and "C1 x y"
    show ?thesis sorry
  next
    fix x y
    assume "B2 x y" and "C2 x y"
    show ?thesis sorry
  qed
end

text {* Here @{text "?thesis"} is used to refer to the unchanged goal
  statement.  *}


subsection {* Rules with cases *}

text {*
  Applying an elimination rule to some goal, leaves that unchanged
  but allows to augment the context in the sub-proof of each case.

  Isar provides some infrastructure to support this:

  \begin{itemize}

  \item native language elements to state eliminations

  \item symbolic case names

  \item method @{method cases} to recover this structure in a
  sub-proof

  \end{itemize}
*}

print_statement exE
print_statement conjE
print_statement disjE

lemma
  assumes A1 and A2  -- {* assumptions *}
  obtains
    (case1)  x y where "B1 x y" and "C1 x y"
  | (case2)  x y where "B2 x y" and "C2 x y"
  sorry


subsubsection {* Example *}

lemma tertium_non_datur:
  obtains
    (T)  A
  | (F)  "\<not> A"
  by blast

notepad
begin
  fix x y :: 'a
  have C
  proof (cases "x = y" rule: tertium_non_datur)
    case T
    from `x = y` show ?thesis sorry
  next
    case F
    from `x \<noteq> y` show ?thesis sorry
  qed
end


subsubsection {* Example *}

text {*
  Isabelle/HOL specification mechanisms (datatype, inductive, etc.)
  provide suitable derived cases rules.
*}

datatype foo = Foo | Bar foo

notepad
begin
  fix x :: foo
  have C
  proof (cases x)
    case Foo
    from `x = Foo` show ?thesis sorry
  next
    case (Bar a)
    from `x = Bar a` show ?thesis sorry
  qed
end


subsection {* Obtaining local contexts *}

text {* A single ``case'' branch may be inlined into Isar proof text
  via @{command obtain}.  This proves @{prop "(\<And>x. B x \<Longrightarrow> thesis) \<Longrightarrow>
  thesis"} on the spot, and augments the context afterwards.  *}

notepad
begin
  fix B :: "'a \<Rightarrow> bool"

  obtain x where "B x" sorry
  note `B x`

  txt {* Conclusions from this context may not mention @{term x} again! *}
  {
    obtain x where "B x" sorry
    from `B x` have C sorry
  }
  note `C`
end

end