unification:

unification:
 - the case

    (? args) v.s. ?(tag:OUT_SCOPE)

   is considered as a pattern-driven delifting.
 - the case ? args v.s. t is reduced to ?[args] v.s. t
 - eta-contraction reimplemented
 - beta-expand no longer used

delift:
 - handle out/in scope tag

ntactics:
 - elim sets the out_scope tag for 1 entry