Definition elt := positive.Definition elt_eq := peq.Inductive tree (A : Type) : Type :=
    | Leaf : tree A
    | Node : tree A -> option A -> tree A -> tree A.Inductive tree (A : Type) : Type :=
    | Leaf : tree A
    | Node : tree A -> option A -> tree A -> tree A.Inductive tree (A : Type) : 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 (A : Type) := (Leaf : t A).Fixpoint get (A : Type) (i : positive) (m : t A) {struct i} : option A :=
    match m with
    | Leaf => None
    | Node l o r =>
        match i with
        | xH => o
        | xO ii => get ii l
        | xI ii => get ii r
        end
    end.Fixpoint set (A : Type) (i : positive) (v : A) (m : t A) {struct i} : t A :=
    match m with
    | Leaf =>
        match i with
        | xH => Node Leaf (Some v) Leaf
        | xO ii => Node (set ii v Leaf) None Leaf
        | xI ii => Node Leaf None (set ii v Leaf)
        end
    | Node l o r =>
        match i with
        | xH => Node l (Some v) r
        | xO ii => Node (set ii v l) o r
        | xI ii => Node l o (set ii v r)
        end
    end.Fixpoint remove (A : Type) (i : positive) (m : t A) {struct i} : t A :=
    match i with
    | xH =>
        match m with
        | Leaf => Leaf
        | Node Leaf o Leaf => Leaf
        | Node l o 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.

Theorem grspec: forall (A: Type) (i j: elt) (m: t A), get i (remove j m) = if elt_eq i j then None else get i m.