fact: Definition tail : forall {A : Type} {n : nat}, t A (S n) -> t A n := fun A n v => match v in t _ m return match m with O => Unit | S k => t A k end with | nil => tt | cons _ _ v' => v' end.
imports: Coq(ssreflect) HoTT(HoTT) HoTT_additions Hierarchy Param_nat