NOT_INTRO : (thm -> thm)

SYNOPSIS
Transforms |- t ==> F into |- ~t.

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

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