DISCH_THEN : (thm_tactic -> tactic)

SYNOPSIS

Undischarges an antecedent of an implication and passes it to a theorem-tactic.

DESCRIBE

DISCH_THEN removes the antecedent and then creates a theorem by ASSUMEing it. This new theorem is passed to the theorem-tactic given as DISCH_THEN's argument. The consequent tactic is then applied. Thus:

   DISCH_THEN f (asl,"t1 ==> t2") = f(ASSUME "t1")(asl,"t2")

For example, if

    A ?- t
   ========  f (ASSUME "u")
    B ?- v

then

    A ?- u ==> t
   ==============  DISCH_THEN f
       B ?- v

Note that DISCH_THEN treats "~u" as "u ==> F".

FAILURE

DISCH_THEN will fail for goals which are not implications or negations.

EXAMPLE

The following shows how DISCH_THEN can be used to preprocess an antecedent before adding it to the assumptions.

    A ?- (x = y) ==> t
   ====================  DISCH_THEN (ASSUME_TAC o SYM)
     A u {y = x} ?- t

In many cases, it is possible to use an antecedent and then throw it away:

    A ?- (x = y) ==> t x
   ======================  DISCH_THEN (\th. PURE_REWRITE_TAC [th])
          A ?- t y

SEEALSO  DISCH,   DISCH_ALL,   DISCH_TAC,   NEG_DISCH,   FILTER_DISCH_TAC,   FILTER_DISCH_THEN,   STRIP_TAC,   UNDISCH,   UNDISCH_ALL,   UNDISCH_TAC