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\DOC CHOOSE_THEN
\TYPE {CHOOSE_THEN : thm_tactical}
\SYNOPSIS
Applies a tactic generated from the body of existentially quantified theorem.
\KEYWORDS
theorem-tactic, existential.
\DESCRIBE
When applied to a theorem-tactic {ttac}, an existentially quantified
theorem {A' |- ?x. t}, and a goal, {CHOOSE_THEN} applies the tactic {ttac
(t[x'/x] |- t[x'/x])} to the goal, where {x'} is a variant of {x} chosen not to
be free in the assumption list of the goal. Thus if:
{
A ?- s1
========= ttac (t[x'/x] |- t[x'/x])
B ?- s2
}
\noindent then
{
A ?- s1
========== CHOOSE_THEN ttac (A' |- ?x. t)
B ?- s2
}
\noindent This is invalid unless {A'} is a subset of {A}.
\FAILURE
Fails unless the given theorem is existentially quantified, or if the
resulting tactic fails when applied to the goal.
\EXAMPLE
This theorem-tactical and its relatives are very useful for using existentially
quantified theorems. For example one might use the inbuilt theorem
{
LESS_ADD_1 = |- !m n. n < m ==> (?p. m = n + (p + 1))
}
\noindent to help solve the goal
{
?- x < y ==> 0 < y * y
}
\noindent by starting with the following tactic
{
DISCH_THEN (CHOOSE_THEN SUBST1_TAC o MATCH_MP LESS_ADD_1)
}
\noindent which reduces the goal to
{
?- 0 < ((x + (p + 1)) * (x + (p + 1)))
}
\noindent which can then be finished off quite easily, by, for example:
{
REWRITE_TAC[ADD_ASSOC; SYM (SPEC_ALL ADD1);
MULT_CLAUSES; ADD_CLAUSES; LESS_0]
}
\SEEALSO
CHOOSE_TAC, X_CHOOSE_THEN.
\ENDDOC
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