Hint Resolve lexfg_app_l.

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