File: NEG_DISCH.doc

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\DOC NEG_DISCH

\TYPE {NEG_DISCH : (term -> thm -> thm)}

\SYNOPSIS
Discharges an assumption, transforming {|- s ==> F} into {|- ~s}.

\KEYWORDS
rule, discharge, assumption, implication, negation.

\DESCRIBE
When applied to a term {s} and a theorem {A |- t}, the inference
rule {NEG_DISCH} returns the theorem {A - {{s}} |- s ==> t}, or if {t}
is just {F}, returns the theorem {A - {{s}} |- ~s}.
{
          A |- F
   --------------------  NEG_DISCH    [special case]
      A - {{s}} |- ~s

          A |- t
   --------------------  NEG_DISCH    [general case]
    A - {{s}} |- s ==> t

}
\FAILURE
Fails unless the supplied term has type {bool}.

\SEEALSO
DISCH, NOT_ELIM, NOT_INTRO.

\ENDDOC