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+theory Preface
+imports Base Main
+begin
+
+chapter {* Preface *}
+
+text {*
+ 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.
+
+ In its pioneering times, the Isabelle/Isar version of the
+ \emph{Proof~General} user interface
+ \cite{proofgeneral,Aspinall:TACAS:2000} has contributed to the
+ success of for interactive theory and proof development in this
+ advanced theorem proving environment, even though it was somewhat
+ biased towards old-style proof scripts. The more recent
+ Isabelle/jEdit Prover IDE \cite{Wenzel:2012} emphasizes the
+ document-oriented approach of Isabelle/Isar again more explicitly.
+
+ \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
+ Isabelle/HOL are described in \partref{part:hol}. Although the main
+ language elements are already provided by the Isabelle/Pure
+ framework, examples given in the generic parts will usually refer to
+ Isabelle/HOL.
+
+ \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 ``@{text
+ "\<^sup>*"}'' 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