Binary tree zipper with basic operations

I am looking for a review of my Binary Tree Zipper implementation on Ocaml. These are the defined operations:

• get (gets the substructure, or the trees after the focus element)
• bempty creates an empty BTZ (binary tree zipper)
• bnext
• bprev
• bdown
• bup
• bupdate
• bremove
• to_btree (BTZ to binary tree)
• from_btree (binary tree to BTZ)

module type BTreeZipper = 
  sig
    type 'a bin_tree
    type 'a context
    type 'a btree_zipper
    exception EmptyZT
    val get : 'a btree_zipper -> 'a bin_tree (* returns the sublist starting at focus position*)
    val bempty : 'a btree_zipper
    val bnext : 'a btree_zipper -> 'a btree_zipper
    val bprev : 'a btree_zipper -> 'a btree_zipper
    val bdown : 'a btree_zipper -> 'a btree_zipper
    val bup : 'a btree_zipper -> 'a btree_zipper
    val bupdate : 'a btree_zipper -> 'a bin_tree -> 'a btree_zipper
    val bremove : 'a btree_zipper -> 'a btree_zipper
    val from_btree : 'a bin_tree -> 'a btree_zipper
    val to_btree : 'a btree_zipper -> 'a bin_tree
  end

module BTreeZipper1 : BTreeZipper =
  struct
    type 'a bin_tree = | Leaf | Node of 'a * 'a bin_tree * 'a bin_tree
    type 'a context = | Top | Context of 'a bin_tree * 'a * 'a context * 'a bin_tree
    type 'a btree_zipper = BTZ of 'a context * 'a bin_tree
    exception EmptyZT
    let get (BTZ(c, t)) = t
    let bempty : 'a btree_zipper = BTZ(Top, Leaf)
    let bnext (BTZ(c, t)) =
      match c with
      | Top -> failwith "root: no right"
      | Context(left, v, up, Leaf) -> failwith "no right"
      | Context(left, v, up, right) -> BTZ (Context(t, v, up, right), right)
    let bprev (BTZ(c, t)) =
      match c with 
      | Top -> failwith "root: no left"
      | Context(Leaf, v, up, right) -> failwith "no left"
      | Context(left, v, up, right) -> BTZ(Context (left, v, up, t), left)
    let bdown (BTZ(c, t)) =
      match t with (* go to left child *)
      | Leaf -> raise EmptyZT (* (BTZ(c, Leaf) => c = Top) => BTZ(Top, Leaf) = bempty*)
      | Node(_,Leaf, Leaf) -> failwith "leaf: no children"
      | Node(v,l,r) -> BTZ (Context(Leaf, v, c, r), l)
    let bup (BTZ(c, t)) =
      match c with
      | Top -> failwith "root: no parent"
      | Context(l, v, u, r) -> BTZ (u, Node(v, l, r))
    let bupdate (BTZ(c, t)) t1 = BTZ(c, t1)
    let bremove (BTZ(c, t)) =
      match c with 
      | Top -> failwith "remove on top"
      | Context(Leaf,v,u,Leaf) -> BTZ(u, Node(v, Leaf, Leaf)) (*has no right or left*)
      | Context(l,v,u, Leaf) -> BTZ(Context(Leaf,v,u,Leaf), l) (*has left*)
      | Context(l,v,u,r) -> BTZ(Context(Leaf,v,u,Leaf), r) (*has right*)
    let from_btree t = BTZ (Top, t)
    let rec to_btree btz =
      match btz with 
      | BTZ(Top, t) -> t
      | BTZ(_,_) -> to_btree (bup btz)
  end

Does it need any improvement?