doc-src/TutorialI/Inductive/document/Mutual.tex

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+++ b/doc-src/TutorialI/Inductive/document/Mutual.tex	Tue Jan 02 12:04:33 2001 +0100
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+%
+\begin{isabellebody}%
+\def\isabellecontext{Mutual}%
+%
+\isamarkupsubsection{Mutual inductive definitions%
+}
+%
+\begin{isamarkuptext}%
+Just as there are datatypes defined by mutual recursion, there are sets defined
+by mutual induction. As a trivial example we consider the even and odd natural numbers:%
+\end{isamarkuptext}%
+\isacommand{consts}\ even\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}nat\ set{\isachardoublequote}\isanewline
+\ \ \ \ \ \ \ odd\ \ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}nat\ set{\isachardoublequote}\isanewline
+\isanewline
+\isacommand{inductive}\ even\ odd\isanewline
+\isakeyword{intros}\isanewline
+zero{\isacharcolon}\ \ {\isachardoublequote}{\isadigit{0}}\ {\isasymin}\ even{\isachardoublequote}\isanewline
+evenI{\isacharcolon}\ {\isachardoublequote}n\ {\isasymin}\ odd\ {\isasymLongrightarrow}\ Suc\ n\ {\isasymin}\ even{\isachardoublequote}\isanewline
+oddI{\isacharcolon}\ \ {\isachardoublequote}n\ {\isasymin}\ even\ {\isasymLongrightarrow}\ Suc\ n\ {\isasymin}\ odd{\isachardoublequote}%
+\begin{isamarkuptext}%
+\noindent
+The simultaneous inductive definition of multiple sets is no different from that
+of a single set, except for induction: just as for mutually recursive datatypes,
+induction needs to involve all the simultaneously defined sets. In the above case,
+the induction rule is called \isa{even{\isacharunderscore}odd{\isachardot}induct} (simply concenate the names
+of the sets involved) and has the conclusion
+\begin{isabelle}%
+\ \ \ \ \ {\isacharparenleft}{\isacharquery}x\ {\isasymin}\ even\ {\isasymlongrightarrow}\ {\isacharquery}P\ {\isacharquery}x{\isacharparenright}\ {\isasymand}\ {\isacharparenleft}{\isacharquery}y\ {\isasymin}\ odd\ {\isasymlongrightarrow}\ {\isacharquery}Q\ {\isacharquery}y{\isacharparenright}%
+\end{isabelle}
+
+If we want to prove that all even numbers are divisible by 2, we have to generalize
+the statement as follows:%
+\end{isamarkuptext}%
+\isacommand{lemma}\ {\isachardoublequote}{\isacharparenleft}m\ {\isasymin}\ even\ {\isasymlongrightarrow}\ {\isadigit{2}}\ dvd\ m{\isacharparenright}\ {\isasymand}\ {\isacharparenleft}n\ {\isasymin}\ odd\ {\isasymlongrightarrow}\ {\isadigit{2}}\ dvd\ {\isacharparenleft}Suc\ n{\isacharparenright}{\isacharparenright}{\isachardoublequote}%
+\begin{isamarkuptxt}%
+\noindent
+The proof is by rule induction. Because of the form of the induction theorem, it is
+applied by \isa{rule} rather than \isa{erule} as for ordinary inductive definitions:%
+\end{isamarkuptxt}%
+\isacommand{apply}{\isacharparenleft}rule\ even{\isacharunderscore}odd{\isachardot}induct{\isacharparenright}%
+\begin{isamarkuptxt}%
+\begin{isabelle}%
+\ {\isadigit{1}}{\isachardot}\ {\isadigit{2}}\ dvd\ {\isadigit{0}}\isanewline
+\ {\isadigit{2}}{\isachardot}\ {\isasymAnd}n{\isachardot}\ {\isasymlbrakk}n\ {\isasymin}\ odd{\isacharsemicolon}\ {\isadigit{2}}\ dvd\ Suc\ n{\isasymrbrakk}\ {\isasymLongrightarrow}\ {\isadigit{2}}\ dvd\ Suc\ n\isanewline
+\ {\isadigit{3}}{\isachardot}\ {\isasymAnd}n{\isachardot}\ {\isasymlbrakk}n\ {\isasymin}\ even{\isacharsemicolon}\ {\isadigit{2}}\ dvd\ n{\isasymrbrakk}\ {\isasymLongrightarrow}\ {\isadigit{2}}\ dvd\ Suc\ {\isacharparenleft}Suc\ n{\isacharparenright}%
+\end{isabelle}
+The first two subgoals are proved by simplification and the final one can be
+proved in the same manner as in \S\ref{sec:rule-induction}
+where the same subgoal was encountered before.
+We do not show the proof script.%
+\end{isamarkuptxt}%
+\end{isabellebody}%
+%%% Local Variables:
+%%% mode: latex
+%%% TeX-master: "root"
+%%% End: