op THEN : tactic * tactic -> tactic
STRUCTURE
Tactical
SYNOPSIS
Applies two tactics in sequence.
DESCRIPTION
If T1 and T2 are tactics, T1 THEN T2 is a tactic which applies T1 to a goal, then applies the tactic T2 to all the subgoals generated. If T1 solves the goal then T2 is never applied.
FAILURE
The application of THEN to a pair of tactics never fails. The resulting tactic fails if T1 fails when applied to the goal, or if T2 does when applied to any of the resulting subgoals.
COMMENTS
Although normally used to sequence tactics which generate a single subgoal, it is worth remembering that it is sometimes useful to apply the same tactic to multiple subgoals; sequences like the following: 
   EQ_TAC THENL [ASM_REWRITE_TAC[], ASM_REWRITE_TAC[]]
can be replaced by the briefer: 
   EQ_TAC THEN ASM_REWRITE_TAC[]

SEEALSO
EVERY, ORELSE, THENL
HOL  Kananaskis-10