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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 | \DOC CONTRAPOS
\TYPE {CONTRAPOS : (thm -> thm)}
\SYNOPSIS
Deduces the contrapositive of an implication.
\KEYWORDS
rule, contrapositive, implication.
\DESCRIBE
When applied to a theorem {A |- s ==> t}, the inference rule {CONTRAPOS}
returns its contrapositive, {A |- ~t ==> ~s}.
{
A |- s ==> t
---------------- CONTRAPOS
A |- ~t ==> ~s
}
\FAILURE
Fails unless the theorem is an implication.
\SEEALSO
CCONTR, CONTR, CONTRAPOS_CONV, NOT_ELIM.
\ENDDOC
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