CONTRAPOS_CONV : term -> thm

SYNOPSIS
Proves the equivalence of an implication and its contrapositive.

DESCRIPTION
When applied to an implication `p ==> q`, the conversion CONTRAPOS_CONV returns the theorem:
   |- (p ==> q) <=> (~q ==> ~p)

FAILURE CONDITIONS
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]

SEE ALSO
CCONTR, CONTR_TAC.