Hint Resolve lexfg_systemSL.

(* Specifications: *)
Theorem lexfg_app_l :
forall a a' b : terms,
e_lexfg _ (e_P1 wt) (e_P2 wt) a a' ->
e_lexfg _ (e_P1 wt) (e_P2 wt) (app a b) (app a' b).