Remark type_expect_incr:
  forall e ty1 ty2 e' te, type_expect e ty1 ty2 = OK e' -> S.satisf te e' -> S.satisf te e.

Lemma type_instr_complete: forall te e i, S.satisf te e -> wt_instr f te i -> exists e', type_instr e i = OK e' /\ S.satisf te e'.