proposition: Lemma appHead_left : forall M (Mapp: isApp M), isFunS (appHead M) -> isFunS (appHead (appBodyL Mapp)).
proof: Proof. intro M; term_inv M. rewrite (appHead_app Tr I); trivial. Qed.
imports: CoLoR(RelExtras ListExtras TermsTyping LogicUtil) Coq(Lia)