Hint Resolve @boxy_exp.

(* Specifications: *)
Lemma boxy_prop {A} `{H : ageable A}:  forall (M: modality) P, reflexive _ (app_mode M) -> boxy M (prop P).
Lemma boxy_TT {A} `{H : ageable A}:  forall (M: modality), reflexive _ (app_mode M) -> boxy M TT.
Lemma boxy_FF {A} `{H : ageable A}:  forall (M: modality), reflexive _ (app_mode M) -> boxy M FF.
Hint Resolve @boxy_TT.