DISCH_THEN : (thm_tactic -> tactic)
STRUCTURE
Thm_cont
SYNOPSIS
Undischarges an antecedent of an implication and passes it to a theorem-tactic.
DESCRIPTION
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
HOL  Kananaskis-13