beta-eta-contract, to respect "first_order_match"'s specification;
Sledgehammer's Skolem cache sometimes failed without the contraction
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\begin{isamarkuptext}%
The \emph{Isabelle} system essentially provides a generic
infrastructure for building deductive systems (programmed in
Standard ML), with a special focus on interactive theorem proving in
higher-order logics. Many years ago, even end-users would refer to
certain ML functions (goal commands, tactics, tacticals etc.) to
pursue their everyday theorem proving tasks.
In contrast \emph{Isar} provides an interpreted language environment
of its own, which has been specifically tailored for the needs of
theory and proof development. Compared to raw ML, the Isabelle/Isar
top-level provides a more robust and comfortable development
platform, with proper support for theory development graphs, managed
transactions with unlimited undo etc. The Isabelle/Isar version of
the \emph{Proof~General} user interface
\cite{proofgeneral,Aspinall:TACAS:2000} provides a decent front-end
for interactive theory and proof development in this advanced
theorem proving environment, even though it is somewhat biased
towards old-style proof scripts.
\medskip Apart from the technical advances over bare-bones ML
programming, the main purpose of the Isar language is to provide a
conceptually different view on machine-checked proofs
\cite{Wenzel:1999:TPHOL,Wenzel-PhD}. \emph{Isar} stands for
\emph{Intelligible semi-automated reasoning}. Drawing from both the
traditions of informal mathematical proof texts and high-level
programming languages, Isar offers a versatile environment for
structured formal proof documents. Thus properly written Isar
proofs become accessible to a broader audience than unstructured
tactic scripts (which typically only provide operational information
for the machine). Writing human-readable proof texts certainly
requires some additional efforts by the writer to achieve a good
presentation, both of formal and informal parts of the text. On the
other hand, human-readable formal texts gain some value in their own
right, independently of the mechanic proof-checking process.
Despite its grand design of structured proof texts, Isar is able to
assimilate the old tactical style as an ``improper'' sub-language.
This provides an easy upgrade path for existing tactic scripts, as
well as some means for interactive experimentation and debugging of
structured proofs. Isabelle/Isar supports a broad range of proof
styles, both readable and unreadable ones.
\medskip The generic Isabelle/Isar framework (see
\chref{ch:isar-framework}) works reasonably well for any Isabelle
object-logic that conforms to the natural deduction view of the
Isabelle/Pure framework. Specific language elements introduced by
the major object-logics are described in \chref{ch:hol}
(Isabelle/HOL), \chref{ch:holcf} (Isabelle/HOLCF), and \chref{ch:zf}
(Isabelle/ZF). The main language elements are already provided by
the Isabelle/Pure framework. Nevertheless, examples given in the
generic parts will usually refer to Isabelle/HOL as well.
\medskip Isar commands may be either \emph{proper} document
constructors, or \emph{improper commands}. Some proof methods and
attributes introduced later are classified as improper as well.
Improper Isar language elements, which are marked by ``\isa{{\isachardoublequote}\isactrlsup {\isacharasterisk}{\isachardoublequote}}'' in the subsequent chapters; they are often helpful
when developing proof documents, but their use is discouraged for
the final human-readable outcome. Typical examples are diagnostic
commands that print terms or theorems according to the current
context; other commands emulate old-style tactical theorem proving.%
\end{isamarkuptext}%
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