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\DOC CONTRAPOS_CONV
\TYPE {CONTRAPOS_CONV : term -> thm}
\SYNOPSIS
Proves the equivalence of an implication and its contrapositive.
\KEYWORDS
conversion, contrapositive, implication.
\DESCRIBE
When applied to an implication {`p ==> q`}, the conversion {CONTRAPOS_CONV}
returns the theorem:
{
|- (p ==> q) <=> (~q ==> ~p)
}
\FAILURE
Fails if applied to a term that is not an implication.
\COMMENTS
The same effect can be had by {GEN_REWRITE_CONV I [GSYM CONTRAPOS_THM]}
\SEEALSO
CCONTR, CONTR_TAC.
\ENDDOC
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