Section Between.
Variables P Q : nat -> Prop.
(** The [between] type expresses the concept [forall i: nat, k <= i < l -> P i.]. *)
Inductive between k : nat -> Prop :=
| bet_emp : between k k
| bet_S : forall l, between k l -> P l -> between k (S l).
Hint Constructors between: arith.

Lemma bet_eq : forall k l, l = k -> between k l.