Thm_cont.UNDISCH_THEN : term -> thm_tactic -> tactic
UNDISCH_THEN t f ([a1,... ai, t, aj, ... an], goal) = f (ASSUME t) ([a1,... ai, aj,... an], goal)
A u {t1} ?- t =============== f (ASSUME t1) B u {t1} ?- v
A u {t1} ?- t =============== UNDISCH_THEN t1 f B ?- v