Ltac TR_AT :=
match goal with
| [ A: (fn_code _)!_ = Some _ , B: reach _ _ |- _ ] =>
generalize (transf_function_at _ _ _ A B); simpl renum_instr; intros
end.

Lemma reach_succ:
forall f pc i s,
f.(fn_code)!pc = Some i -> In s (successors_instr i) ->
reach f pc -> reach f s.