约束/保护OCaml中的类型(Constraint/Guard a type in OCaml)

 给定是一种用于构造二叉树的类型： 
 
type tree = Leaf of int | Node of int * tree * tree
 
 现在假设我们想通过类型表示二叉树包含一个元素为零的节点，即，我想表达以下形式的东西： 
 
let rec p x = match x with 
            | Leaf(y) -> y = 0
            | Node(y,l,r) -> y = 0 || (p l) || (p r)

type zerotree = ZeroTree of t:tree where p(t)
 
 这意味着，每当我有一个ZeroTree类型的树，那么我可以确定树包含一个元素为零的节点，即谓词p成立。 
 
 在OCaml中有类似的东西吗？ 

Given is a type in order to construct a binary tree:
 
type tree = Leaf of int | Node of int * tree * tree
 
Now assume we want to express via types that a binary tree contains a node with an element zero, i.e., I would like to express something of the following form:
 
let rec p x = match x with 
            | Leaf(y) -> y = 0
            | Node(y,l,r) -> y = 0 || (p l) || (p r)

type zerotree = ZeroTree of t:tree where p(t)
 
That means, whenever I have a tree of type ZeroTree, then I can be sure that the tree contains a node with an element zero, i.e., the predicate p holds.
 
Is something like that expressible in OCaml?

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