Proof using.
induction l; intros; simpl in *; intuition.
- eauto.
- find_copy_apply_hyp_hyp. break_exists_exists. intuition.
right. intuition. congruence.
Qed.

(* Specifications: *)
Lemma before_func_prefix :
forall l l',
Prefix l l' ->
before_func f g l ->
before_func f g l'.