Module E:=PositiveOrderedTypeBits.Module ME:=KeyOrderedType E.Definition key := positive : Type.#[universes(template)]
  Inductive tree (A : Type) :=
    | Leaf : tree A
    | Node : tree A -> option A -> tree A -> tree A.#[universes(template)]
  Inductive tree (A : Type) :=
    | Leaf : tree A
    | Node : tree A -> option A -> tree A -> tree A.#[universes(template)]
  Inductive tree (A : Type) :=
    | Leaf : tree A
    | Node : tree A -> option A -> tree A -> tree A.Scheme tree_ind := Induction for tree Sort Prop.Definition t := tree.Definition empty : t A := Leaf.Fixpoint is_empty (m : t A) : bool :=
   match m with
    | Leaf => true
    | Node l None r => (is_empty l) && (is_empty r)
    | _ => false
   end.Fixpoint find (i : key) (m : t A) : option A :=
    match m with
    | Leaf => None
    | Node l o r =>
        match i with
        | xH => o
        | xO ii => find ii l
        | xI ii => find ii r
        end
    end.Fixpoint mem (i : key) (m : t A) : bool :=
    match m with
    | Leaf => false
    | Node l o r =>
        match i with
        | xH => match o with None => false | _ => true end
        | xO ii => mem ii l
        | xI ii => mem ii r
        end
    end.Fixpoint add (i : key) (v : A) (m : t A) : t A :=
    match m with
    | Leaf =>
        match i with
        | xH => Node Leaf (Some v) Leaf
        | xO ii => Node (add ii v Leaf) None Leaf
        | xI ii => Node Leaf None (add ii v Leaf)
        end
    | Node l o r =>
        match i with
        | xH => Node l (Some v) r
        | xO ii => Node (add ii v l) o r
        | xI ii => Node l o (add ii v r)
        end
    end.Fixpoint remove (i : key) (m : t A) : t A :=
    match i with
    | xH =>
        match m with
        | Leaf => Leaf
        | Node Leaf _ Leaf => Leaf
        | Node l _ r => Node l None r
        end
    | xO ii =>
        match m with
        | Leaf => Leaf
        | Node l None Leaf =>
            match remove ii l with
            | Leaf => Leaf
            | mm => Node mm None Leaf
            end
        | Node l o r => Node (remove ii l) o r
        end
    | xI ii =>
        match m with
        | Leaf => Leaf
        | Node Leaf None r =>
            match remove ii r with
            | Leaf => Leaf
            | mm => Node Leaf None mm
            end
        | Node l o r => Node l o (remove ii r)
        end
    end.(** [elements] *)

    Fixpoint xelements (m : t A) (i : key) : list (key * A) :=
      match m with
      | Leaf => nil
      | Node l None r =>
          (xelements l (append i (xO xH))) ++ (xelements r (append i (xI xH)))
      | Node l (Some x) r =>
          (xelements l (append i (xO xH)))
          ++ ((i, x) :: xelements r (append i (xI xH)))
      end.(* Note: function [xelements] above is inefficient.  We should apply
     deforestation to it, but that makes the proofs even harder. *)

  Definition elements (m : t A) := xelements m xH.(** [cardinal] *)

  Fixpoint cardinal (m : t A) : nat :=
    match m with
      | Leaf => 0%nat
      | Node l None r => (cardinal l + cardinal r)%nat
      | Node l (Some _) r => S (cardinal l + cardinal r)
    end.

Theorem grs: forall (i: key) (m: t A), find i (remove i m) = None.