File: NOT_INTRO.doc

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\DOC NOT_INTRO

\TYPE {NOT_INTRO : (thm -> thm)}

\SYNOPSIS
Transforms {|- t ==> F} into {|- ~t}.

\KEYWORDS
rule, negation, implication.

\DESCRIBE
When applied to a theorem {A |- t ==> F}, the inference rule {NOT_INTRO}
returns the theorem {A |- ~t}.
{
    A |- t ==> F
   --------------  NOT_INTRO
      A |- ~t
}
\FAILURE
Fails unless the theorem has an implicative conclusion with {F}
as the consequent.

\SEEALSO
IMP_ELIM, NOT_ELIM.

\ENDDOC