%

\begin{isabellebody}%

\def\isabellecontext{AB}%

%

\isamarkupsection{Case Study: A Context Free Grammar%

}

%

\begin{isamarkuptext}%

\label{sec:CFG}

Grammars are nothing but shorthands for inductive definitions of nonterminals

which represent sets of strings. For example, the production

$A \to B c$ is short for

\[ w \in B \Longrightarrow wc \in A \]

This section demonstrates this idea with an example

due to Hopcroft and Ullman, a grammar for generating all words with an

equal number of $a$'s and~$b$'s:

\begin{eqnarray}

S &\to& \epsilon \mid b A \mid a B \nonumber\\

A &\to& a S \mid b A A \nonumber\\

B &\to& b S \mid a B B \nonumber

\end{eqnarray}

At the end we say a few words about the relationship between

the original proof \cite[p.\ts81]{HopcroftUllman} and our formal version.

We start by fixing the alphabet, which consists only of \isa{a}'s

and~\isa{b}'s:%

\end{isamarkuptext}%

\isacommand{datatype}\ alfa\ {\isacharequal}\ a\ {\isacharbar}\ b%

\begin{isamarkuptext}%

\noindent

For convenience we include the following easy lemmas as simplification rules:%

\end{isamarkuptext}%

\isacommand{lemma}\ {\isacharbrackleft}simp{\isacharbrackright}{\isacharcolon}\ {\isachardoublequote}{\isacharparenleft}x\ {\isasymnoteq}\ a{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}x\ {\isacharequal}\ b{\isacharparenright}\ {\isasymand}\ {\isacharparenleft}x\ {\isasymnoteq}\ b{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}x\ {\isacharequal}\ a{\isacharparenright}{\isachardoublequote}\isanewline

\isacommand{by}\ {\isacharparenleft}case{\isacharunderscore}tac\ x{\isacharcomma}\ auto{\isacharparenright}%

\begin{isamarkuptext}%

\noindent

Words over this alphabet are of type \isa{alfa\ list}, and

the three nonterminals are declared as sets of such words:%

\end{isamarkuptext}%

\isacommand{consts}\ S\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}alfa\ list\ set{\isachardoublequote}\isanewline

\ \ \ \ \ \ \ A\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}alfa\ list\ set{\isachardoublequote}\isanewline

\ \ \ \ \ \ \ B\ {\isacharcolon}{\isacharcolon}\ {\isachardoublequote}alfa\ list\ set{\isachardoublequote}%

\begin{isamarkuptext}%

\noindent

The productions above are recast as a \emph{mutual} inductive

definition\index{inductive definition!simultaneous}

of \isa{S}, \isa{A} and~\isa{B}:%

\end{isamarkuptext}%

\isacommand{inductive}\ S\ A\ B\isanewline

\isakeyword{intros}\isanewline

\ \ {\isachardoublequote}{\isacharbrackleft}{\isacharbrackright}\ {\isasymin}\ S{\isachardoublequote}\isanewline

\ \ {\isachardoublequote}w\ {\isasymin}\ A\ {\isasymLongrightarrow}\ b{\isacharhash}w\ {\isasymin}\ S{\isachardoublequote}\isanewline

\ \ {\isachardoublequote}w\ {\isasymin}\ B\ {\isasymLongrightarrow}\ a{\isacharhash}w\ {\isasymin}\ S{\isachardoublequote}\isanewline

\isanewline

\ \ {\isachardoublequote}w\ {\isasymin}\ S\ \ \ \ \ \ \ \ {\isasymLongrightarrow}\ a{\isacharhash}w\ \ \ {\isasymin}\ A{\isachardoublequote}\isanewline

\ \ {\isachardoublequote}{\isasymlbrakk}\ v{\isasymin}A{\isacharsemicolon}\ w{\isasymin}A\ {\isasymrbrakk}\ {\isasymLongrightarrow}\ b{\isacharhash}v{\isacharat}w\ {\isasymin}\ A{\isachardoublequote}\isanewline

\isanewline

\ \ {\isachardoublequote}w\ {\isasymin}\ S\ \ \ \ \ \ \ \ \ \ \ \ {\isasymLongrightarrow}\ b{\isacharhash}w\ \ \ {\isasymin}\ B{\isachardoublequote}\isanewline

\ \ {\isachardoublequote}{\isasymlbrakk}\ v\ {\isasymin}\ B{\isacharsemicolon}\ w\ {\isasymin}\ B\ {\isasymrbrakk}\ {\isasymLongrightarrow}\ a{\isacharhash}v{\isacharat}w\ {\isasymin}\ B{\isachardoublequote}%

\begin{isamarkuptext}%

\noindent

First we show that all words in \isa{S} contain the same number of \isa{a}'s and \isa{b}'s. Since the definition of \isa{S} is by mutual

induction, so is the proof: we show at the same time that all words in

\isa{A} contain one more \isa{a} than \isa{b} and all words in \isa{B} contains one more \isa{b} than \isa{a}.%

\end{isamarkuptext}%

\isacommand{lemma}\ correctness{\isacharcolon}\isanewline

\ \ {\isachardoublequote}{\isacharparenleft}w\ {\isasymin}\ S\ {\isasymlongrightarrow}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}a{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}b{\isacharbrackright}{\isacharparenright}\ \ \ \ \ {\isasymand}\isanewline

\ \ \ {\isacharparenleft}w\ {\isasymin}\ A\ {\isasymlongrightarrow}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}a{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}b{\isacharbrackright}\ {\isacharplus}\ {\isadigit{1}}{\isacharparenright}\ {\isasymand}\isanewline

\ \ \ {\isacharparenleft}w\ {\isasymin}\ B\ {\isasymlongrightarrow}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}b{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}a{\isacharbrackright}\ {\isacharplus}\ {\isadigit{1}}{\isacharparenright}{\isachardoublequote}%

\begin{isamarkuptxt}%

\noindent

These propositions are expressed with the help of the predefined \isa{filter} function on lists, which has the convenient syntax \isa{{\isacharbrackleft}x{\isasymin}xs{\isachardot}\ P\ x{\isacharbrackright}}, the list of all elements \isa{x} in \isa{xs} such that \isa{P\ x}

holds. Remember that on lists \isa{size} and \isa{length} are synonymous.

The proof itself is by rule induction and afterwards automatic:%

\end{isamarkuptxt}%

\isacommand{by}\ {\isacharparenleft}rule\ S{\isacharunderscore}A{\isacharunderscore}B{\isachardot}induct{\isacharcomma}\ auto{\isacharparenright}%

\begin{isamarkuptext}%

\noindent

This may seem surprising at first, and is indeed an indication of the power

of inductive definitions. But it is also quite straightforward. For example,

consider the production $A \to b A A$: if $v,w \in A$ and the elements of $A$

contain one more $a$ than~$b$'s, then $bvw$ must again contain one more $a$

than~$b$'s.

As usual, the correctness of syntactic descriptions is easy, but completeness

is hard: does \isa{S} contain \emph{all} words with an equal number of

\isa{a}'s and \isa{b}'s? It turns out that this proof requires the

following lemma: every string with two more \isa{a}'s than \isa{b}'s can be cut somewhere such that each half has one more \isa{a} than

\isa{b}. This is best seen by imagining counting the difference between the

number of \isa{a}'s and \isa{b}'s starting at the left end of the

word. We start with 0 and end (at the right end) with 2. Since each move to the

right increases or decreases the difference by 1, we must have passed through

1 on our way from 0 to 2. Formally, we appeal to the following discrete

intermediate value theorem \isa{nat{\isadigit{0}}{\isacharunderscore}intermed{\isacharunderscore}int{\isacharunderscore}val}

\begin{isabelle}%

\ \ \ \ \ {\isasymlbrakk}{\isasymforall}i{\isachardot}\ i\ {\isacharless}\ n\ {\isasymlongrightarrow}\ {\isasymbar}f\ {\isacharparenleft}i\ {\isacharplus}\ {\isadigit{1}}{\isacharparenright}\ {\isacharminus}\ f\ i{\isasymbar}\ {\isasymle}\ {\isacharhash}{\isadigit{1}}{\isacharsemicolon}\ f\ {\isadigit{0}}\ {\isasymle}\ k{\isacharsemicolon}\ k\ {\isasymle}\ f\ n{\isasymrbrakk}\isanewline

\isaindent{\ \ \ \ \ }{\isasymLongrightarrow}\ {\isasymexists}i{\isachardot}\ i\ {\isasymle}\ n\ {\isasymand}\ f\ i\ {\isacharequal}\ k%

\end{isabelle}

where \isa{f} is of type \isa{nat\ {\isasymRightarrow}\ int}, \isa{int} are the integers,

\isa{{\isasymbar}{\isachardot}{\isasymbar}} is the absolute value function, and \isa{{\isacharhash}{\isadigit{1}}} is the

integer 1 (see \S\ref{sec:numbers}).

First we show that our specific function, the difference between the

numbers of \isa{a}'s and \isa{b}'s, does indeed only change by 1 in every

move to the right. At this point we also start generalizing from \isa{a}'s

and \isa{b}'s to an arbitrary property \isa{P}. Otherwise we would have

to prove the desired lemma twice, once as stated above and once with the

roles of \isa{a}'s and \isa{b}'s interchanged.%

\end{isamarkuptext}%

\isacommand{lemma}\ step{\isadigit{1}}{\isacharcolon}\ {\isachardoublequote}{\isasymforall}i\ {\isacharless}\ size\ w{\isachardot}\isanewline

\ \ {\isasymbar}{\isacharparenleft}int{\isacharparenleft}size{\isacharbrackleft}x{\isasymin}take\ {\isacharparenleft}i{\isacharplus}{\isadigit{1}}{\isacharparenright}\ w{\isachardot}\ P\ x{\isacharbrackright}{\isacharparenright}{\isacharminus}int{\isacharparenleft}size{\isacharbrackleft}x{\isasymin}take\ {\isacharparenleft}i{\isacharplus}{\isadigit{1}}{\isacharparenright}\ w{\isachardot}\ {\isasymnot}P\ x{\isacharbrackright}{\isacharparenright}{\isacharparenright}\isanewline

\ \ \ {\isacharminus}\ {\isacharparenleft}int{\isacharparenleft}size{\isacharbrackleft}x{\isasymin}take\ i\ w{\isachardot}\ P\ x{\isacharbrackright}{\isacharparenright}{\isacharminus}int{\isacharparenleft}size{\isacharbrackleft}x{\isasymin}take\ i\ w{\isachardot}\ {\isasymnot}P\ x{\isacharbrackright}{\isacharparenright}{\isacharparenright}{\isasymbar}\ {\isasymle}\ {\isacharhash}{\isadigit{1}}{\isachardoublequote}%

\begin{isamarkuptxt}%

\noindent

The lemma is a bit hard to read because of the coercion function

\isa{int\ {\isacharcolon}{\isacharcolon}\ nat\ {\isasymRightarrow}\ int}. It is required because \isa{size} returns

a natural number, but subtraction on type~\isa{nat} will do the wrong thing.

Function \isa{take} is predefined and \isa{take\ i\ xs} is the prefix of

length \isa{i} of \isa{xs}; below we also need \isa{drop\ i\ xs}, which

is what remains after that prefix has been dropped from \isa{xs}.

The proof is by induction on \isa{w}, with a trivial base case, and a not

so trivial induction step. Since it is essentially just arithmetic, we do not

discuss it.%

\end{isamarkuptxt}%

\isacommand{apply}{\isacharparenleft}induct\ w{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}simp{\isacharparenright}\isanewline

\isacommand{by}{\isacharparenleft}force\ simp\ add{\isacharcolon}zabs{\isacharunderscore}def\ take{\isacharunderscore}Cons\ split{\isacharcolon}nat{\isachardot}split\ if{\isacharunderscore}splits{\isacharparenright}%

\begin{isamarkuptext}%

Finally we come to the above mentioned lemma about cutting in half a word with two

more elements of one sort than of the other sort:%

\end{isamarkuptext}%

\isacommand{lemma}\ part{\isadigit{1}}{\isacharcolon}\isanewline

\ {\isachardoublequote}size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ P\ x{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ {\isasymnot}P\ x{\isacharbrackright}{\isacharplus}{\isadigit{2}}\ {\isasymLongrightarrow}\isanewline

\ \ {\isasymexists}i{\isasymle}size\ w{\isachardot}\ size{\isacharbrackleft}x{\isasymin}take\ i\ w{\isachardot}\ P\ x{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}take\ i\ w{\isachardot}\ {\isasymnot}P\ x{\isacharbrackright}{\isacharplus}{\isadigit{1}}{\isachardoublequote}%

\begin{isamarkuptxt}%

\noindent

This is proved by \isa{force} with the help of the intermediate value theorem,

instantiated appropriately and with its first premise disposed of by lemma

\isa{step{\isadigit{1}}}:%

\end{isamarkuptxt}%

\isacommand{apply}{\isacharparenleft}insert\ nat{\isadigit{0}}{\isacharunderscore}intermed{\isacharunderscore}int{\isacharunderscore}val{\isacharbrackleft}OF\ step{\isadigit{1}}{\isacharcomma}\ of\ {\isachardoublequote}P{\isachardoublequote}\ {\isachardoublequote}w{\isachardoublequote}\ {\isachardoublequote}{\isacharhash}{\isadigit{1}}{\isachardoublequote}{\isacharbrackright}{\isacharparenright}\isanewline

\isacommand{by}\ force%

\begin{isamarkuptext}%

\noindent

Lemma \isa{part{\isadigit{1}}} tells us only about the prefix \isa{take\ i\ w}.

An easy lemma deals with the suffix \isa{drop\ i\ w}:%

\end{isamarkuptext}%

\isacommand{lemma}\ part{\isadigit{2}}{\isacharcolon}\isanewline

\ \ {\isachardoublequote}{\isasymlbrakk}size{\isacharbrackleft}x{\isasymin}take\ i\ w\ {\isacharat}\ drop\ i\ w{\isachardot}\ P\ x{\isacharbrackright}\ {\isacharequal}\isanewline

\ \ \ \ size{\isacharbrackleft}x{\isasymin}take\ i\ w\ {\isacharat}\ drop\ i\ w{\isachardot}\ {\isasymnot}P\ x{\isacharbrackright}{\isacharplus}{\isadigit{2}}{\isacharsemicolon}\isanewline

\ \ \ \ size{\isacharbrackleft}x{\isasymin}take\ i\ w{\isachardot}\ P\ x{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}take\ i\ w{\isachardot}\ {\isasymnot}P\ x{\isacharbrackright}{\isacharplus}{\isadigit{1}}{\isasymrbrakk}\isanewline

\ \ \ {\isasymLongrightarrow}\ size{\isacharbrackleft}x{\isasymin}drop\ i\ w{\isachardot}\ P\ x{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}drop\ i\ w{\isachardot}\ {\isasymnot}P\ x{\isacharbrackright}{\isacharplus}{\isadigit{1}}{\isachardoublequote}\isanewline

\isacommand{by}{\isacharparenleft}simp\ del{\isacharcolon}append{\isacharunderscore}take{\isacharunderscore}drop{\isacharunderscore}id{\isacharparenright}%

\begin{isamarkuptext}%

\noindent

In the proof, we have had to disable a normally useful lemma:

\begin{isabelle}

\isa{take\ n\ xs\ {\isacharat}\ drop\ n\ xs\ {\isacharequal}\ xs}

\rulename{append_take_drop_id}

\end{isabelle}

To dispose of trivial cases automatically, the rules of the inductive

definition are declared simplification rules:%

\end{isamarkuptext}%

\isacommand{declare}\ S{\isacharunderscore}A{\isacharunderscore}B{\isachardot}intros{\isacharbrackleft}simp{\isacharbrackright}%

\begin{isamarkuptext}%

\noindent

This could have been done earlier but was not necessary so far.

The completeness theorem tells us that if a word has the same number of

\isa{a}'s and \isa{b}'s, then it is in \isa{S}, and similarly 

for \isa{A} and \isa{B}:%

\end{isamarkuptext}%

\isacommand{theorem}\ completeness{\isacharcolon}\isanewline

\ \ {\isachardoublequote}{\isacharparenleft}size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}a{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}b{\isacharbrackright}\ \ \ \ \ {\isasymlongrightarrow}\ w\ {\isasymin}\ S{\isacharparenright}\ {\isasymand}\isanewline

\ \ \ {\isacharparenleft}size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}a{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}b{\isacharbrackright}\ {\isacharplus}\ {\isadigit{1}}\ {\isasymlongrightarrow}\ w\ {\isasymin}\ A{\isacharparenright}\ {\isasymand}\isanewline

\ \ \ {\isacharparenleft}size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}b{\isacharbrackright}\ {\isacharequal}\ size{\isacharbrackleft}x{\isasymin}w{\isachardot}\ x{\isacharequal}a{\isacharbrackright}\ {\isacharplus}\ {\isadigit{1}}\ {\isasymlongrightarrow}\ w\ {\isasymin}\ B{\isacharparenright}{\isachardoublequote}%

\begin{isamarkuptxt}%

\noindent

The proof is by induction on \isa{w}. Structural induction would fail here

because, as we can see from the grammar, we need to make bigger steps than

merely appending a single letter at the front. Hence we induct on the length

of \isa{w}, using the induction rule \isa{length{\isacharunderscore}induct}:%

\end{isamarkuptxt}%

\isacommand{apply}{\isacharparenleft}induct{\isacharunderscore}tac\ w\ rule{\isacharcolon}\ length{\isacharunderscore}induct{\isacharparenright}%

\begin{isamarkuptxt}%

\noindent

The \isa{rule} parameter tells \isa{induct{\isacharunderscore}tac} explicitly which induction

rule to use. For details see \S\ref{sec:complete-ind} below.

In this case the result is that we may assume the lemma already

holds for all words shorter than \isa{w}.

The proof continues with a case distinction on \isa{w},

i.e.\ if \isa{w} is empty or not.%

\end{isamarkuptxt}%

\isacommand{apply}{\isacharparenleft}case{\isacharunderscore}tac\ w{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}simp{\isacharunderscore}all{\isacharparenright}%

\begin{isamarkuptxt}%

\noindent

Simplification disposes of the base case and leaves only two step

cases to be proved:

if \isa{w\ {\isacharequal}\ a\ {\isacharhash}\ v} and \begin{isabelle}%

\ \ \ \ \ length\ {\isacharbrackleft}x{\isasymin}v\ {\isachardot}\ x\ {\isacharequal}\ a{\isacharbrackright}\ {\isacharequal}\ length\ {\isacharbrackleft}x{\isasymin}v\ {\isachardot}\ x\ {\isacharequal}\ b{\isacharbrackright}\ {\isacharplus}\ {\isadigit{2}}%

\end{isabelle} then

\isa{b\ {\isacharhash}\ v\ {\isasymin}\ A}, and similarly for \isa{w\ {\isacharequal}\ b\ {\isacharhash}\ v}.

We only consider the first case in detail.

After breaking the conjuction up into two cases, we can apply

\isa{part{\isadigit{1}}} to the assumption that \isa{w} contains two more \isa{a}'s than \isa{b}'s.%

\end{isamarkuptxt}%

\isacommand{apply}{\isacharparenleft}rule\ conjI{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}clarify{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}frule\ part{\isadigit{1}}{\isacharbrackleft}of\ {\isachardoublequote}{\isasymlambda}x{\isachardot}\ x{\isacharequal}a{\isachardoublequote}{\isacharcomma}\ simplified{\isacharbrackright}{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}erule\ exE{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}erule\ conjE{\isacharparenright}%

\begin{isamarkuptxt}%

\noindent

This yields an index \isa{i\ {\isasymle}\ length\ v} such that

\begin{isabelle}%

\ \ \ \ \ length\ {\isacharbrackleft}x{\isasymin}take\ i\ v\ {\isachardot}\ x\ {\isacharequal}\ a{\isacharbrackright}\ {\isacharequal}\ length\ {\isacharbrackleft}x{\isasymin}take\ i\ v\ {\isachardot}\ x\ {\isacharequal}\ b{\isacharbrackright}\ {\isacharplus}\ {\isadigit{1}}%

\end{isabelle}

With the help of \isa{part{\isadigit{2}}} it follows that

\begin{isabelle}%

\ \ \ \ \ length\ {\isacharbrackleft}x{\isasymin}drop\ i\ v\ {\isachardot}\ x\ {\isacharequal}\ a{\isacharbrackright}\ {\isacharequal}\ length\ {\isacharbrackleft}x{\isasymin}drop\ i\ v\ {\isachardot}\ x\ {\isacharequal}\ b{\isacharbrackright}\ {\isacharplus}\ {\isadigit{1}}%

\end{isabelle}%

\end{isamarkuptxt}%

\ \isacommand{apply}{\isacharparenleft}drule\ part{\isadigit{2}}{\isacharbrackleft}of\ {\isachardoublequote}{\isasymlambda}x{\isachardot}\ x{\isacharequal}a{\isachardoublequote}{\isacharcomma}\ simplified{\isacharbrackright}{\isacharparenright}\isanewline

\ \ \isacommand{apply}{\isacharparenleft}assumption{\isacharparenright}%

\begin{isamarkuptxt}%

\noindent

Now it is time to decompose \isa{v} in the conclusion \isa{b\ {\isacharhash}\ v\ {\isasymin}\ A}

into \isa{take\ i\ v\ {\isacharat}\ drop\ i\ v},

after which the appropriate rule of the grammar reduces the goal

to the two subgoals \isa{take\ i\ v\ {\isasymin}\ A} and \isa{drop\ i\ v\ {\isasymin}\ A}:%

\end{isamarkuptxt}%

\ \isacommand{apply}{\isacharparenleft}rule{\isacharunderscore}tac\ n{\isadigit{1}}{\isacharequal}i\ \isakeyword{and}\ t{\isacharequal}v\ \isakeyword{in}\ subst{\isacharbrackleft}OF\ append{\isacharunderscore}take{\isacharunderscore}drop{\isacharunderscore}id{\isacharbrackright}{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}rule\ S{\isacharunderscore}A{\isacharunderscore}B{\isachardot}intros{\isacharparenright}%

\begin{isamarkuptxt}%

\noindent

Both subgoals follow from the induction hypothesis because both \isa{take\ i\ v} and \isa{drop\ i\ v} are shorter than \isa{w}:%

\end{isamarkuptxt}%

\ \ \isacommand{apply}{\isacharparenleft}force\ simp\ add{\isacharcolon}\ min{\isacharunderscore}less{\isacharunderscore}iff{\isacharunderscore}disj{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}force\ split\ add{\isacharcolon}\ nat{\isacharunderscore}diff{\isacharunderscore}split{\isacharparenright}%

\begin{isamarkuptxt}%

\noindent

Note that the variables \isa{n{\isadigit{1}}} and \isa{t} referred to in the

substitution step above come from the derived theorem \isa{subst{\isacharbrackleft}OF\ append{\isacharunderscore}take{\isacharunderscore}drop{\isacharunderscore}id{\isacharbrackright}}.

The case \isa{w\ {\isacharequal}\ b\ {\isacharhash}\ v} is proved analogously:%

\end{isamarkuptxt}%

\isacommand{apply}{\isacharparenleft}clarify{\isacharparenright}\isanewline

\isacommand{apply}{\isacharparenleft}frule\ part{\isadigit{1}}{\isacharbrackleft}of\ {\isachardoublequote}{\isasymlambda}x{\isachardot}\ x{\isacharequal}b{\isachardoublequote}{\isacharcomma}\ simplified{\isacharbrackright}{\isacharparenright}\isanewline

\isacommand{apply}{\isacharparenleft}erule\ exE{\isacharparenright}\isanewline

\isacommand{apply}{\isacharparenleft}erule\ conjE{\isacharparenright}\isanewline

\isacommand{apply}{\isacharparenleft}drule\ part{\isadigit{2}}{\isacharbrackleft}of\ {\isachardoublequote}{\isasymlambda}x{\isachardot}\ x{\isacharequal}b{\isachardoublequote}{\isacharcomma}\ simplified{\isacharbrackright}{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}assumption{\isacharparenright}\isanewline

\isacommand{apply}{\isacharparenleft}rule{\isacharunderscore}tac\ n{\isadigit{1}}{\isacharequal}i\ \isakeyword{and}\ t{\isacharequal}v\ \isakeyword{in}\ subst{\isacharbrackleft}OF\ append{\isacharunderscore}take{\isacharunderscore}drop{\isacharunderscore}id{\isacharbrackright}{\isacharparenright}\isanewline

\isacommand{apply}{\isacharparenleft}rule\ S{\isacharunderscore}A{\isacharunderscore}B{\isachardot}intros{\isacharparenright}\isanewline

\ \isacommand{apply}{\isacharparenleft}force\ simp\ add{\isacharcolon}min{\isacharunderscore}less{\isacharunderscore}iff{\isacharunderscore}disj{\isacharparenright}\isanewline

\isacommand{by}{\isacharparenleft}force\ simp\ add{\isacharcolon}min{\isacharunderscore}less{\isacharunderscore}iff{\isacharunderscore}disj\ split\ add{\isacharcolon}\ nat{\isacharunderscore}diff{\isacharunderscore}split{\isacharparenright}%

\begin{isamarkuptext}%

We conclude this section with a comparison of our proof with 

Hopcroft and Ullman's \cite[p.\ts81]{HopcroftUllman}. For a start, the texbook

grammar, for no good reason, excludes the empty word.  That complicates

matters just a little bit because we now have 8 instead of our 7 productions.

More importantly, the proof itself is different: rather than

separating the two directions, they perform one induction on the

length of a word. This deprives them of the beauty of rule induction,

and in the easy direction (correctness) their reasoning is more

detailed than our \isa{auto}. For the hard part (completeness), they

consider just one of the cases that our \isa{simp{\isacharunderscore}all} disposes of

automatically. Then they conclude the proof by saying about the

remaining cases: ``We do this in a manner similar to our method of

proof for part (1); this part is left to the reader''. But this is

precisely the part that requires the intermediate value theorem and

thus is not at all similar to the other cases (which are automatic in

Isabelle). The authors are at least cavalier about this point and may

even have overlooked the slight difficulty lurking in the omitted

cases. This is not atypical for pen-and-paper proofs, once analysed in

detail.%

\end{isamarkuptext}%

\end{isabellebody}%

%%% Local Variables:

%%% mode: latex

%%% TeX-master: "root"

%%% End: