(* 
  A list of trees is a cover of a tree if all the trees in the
  list delimit a frontier in the tree
*)
Inductive cover : list (btree A) -> btree A -> Prop :=
  | cover_one : forall t, cover (t :: nil) t
  | cover_node :
      forall l1 l2 t1 t2 t3,
      permutation l1 (t1 :: t2 :: l2) ->
      cover (node t1 t2 :: l2) t3 -> cover l1 t3.(* 
  A list of trees is a cover of a tree if all the trees in the
  list delimit a frontier in the tree
*)
Inductive cover : list (btree A) -> btree A -> Prop :=
  | cover_one : forall t, cover (t :: nil) t
  | cover_node :
      forall l1 l2 t1 t2 t3,
      permutation l1 (t1 :: t2 :: l2) ->
      cover (node t1 t2 :: l2) t3 -> cover l1 t3.(* 
  A list of trees is a cover of a tree if all the trees in the
  list delimit a frontier in the tree
*)
Inductive cover : list (btree A) -> btree A -> Prop :=
  | cover_one : forall t, cover (t :: nil) t
  | cover_node :
      forall l1 l2 t1 t2 t3,
      permutation l1 (t1 :: t2 :: l2) ->
      cover (node t1 t2 :: l2) t3 -> cover l1 t3.

Theorem one_cover_ex : forall l : list (btree A), l <> nil -> exists t : btree A, cover l t.