Section defs.
Variable A : Type.
Variable R : A -> A -> Prop.
(** Locally sorted: consecutive elements of the list are ordered *)
Inductive LocallySorted : list A -> Prop :=
| LSorted_nil : LocallySorted []
| LSorted_cons1 a : LocallySorted [a]
| LSorted_consn a b l :
LocallySorted (b :: l) -> R a b -> LocallySorted (a :: b :: l).
(** Alternative two-step definition of being locally sorted *)
Inductive HdRel a : list A -> Prop :=
| HdRel_nil : HdRel a []
| HdRel_cons b l : R a b -> HdRel a (b :: l).
Inductive Sorted : list A -> Prop :=
| Sorted_nil : Sorted []
| Sorted_cons a l : Sorted l -> HdRel a l -> Sorted (a :: l).

Lemma Sorted_inv :
forall a l, Sorted (a :: l) -> Sorted l /\ HdRel a l.