proposition: Lemma is_pair_inr {t} (h : hereditary_type t) : forall t', is_pair h = inr t' -> fst h = t'.
proof: Proof. let elim := constr:(fun_elim (f:=@is_pair)) in apply elim; simpl; intros; try congruence. Qed.
imports: Equations(Equations) Lia List Utf8 Arith Lia Relations Setoid Arith Wf_nat