// A tree type, where nodes can have zero, one, or two children.
type Tree = union {
    Leaf: int;
    Node1: struct {value: int; child: Tree};
    Node2: struct {value: int; child1: Tree; child2: Tree}
};

// A (non-empty) list, where each node can have zero or one child.
type List = union {
    Leaf: int;
    Node1: struct {value: int; child: List}
};

// Check whether the given tree only has one element.
fun hasSize1(t: Tree): bool =
    match t with {
        Leaf{_} -> true;
        Node1{_} -> false;
        Node2{_} -> false
    };

// Here, 't' can be given type Tree or List (both type-check)
let t: Tree = Node1{struct{value = 1;
                           child = Node1{struct{value = 2;
                                                child = Leaf{3}}}}};

// If 't' above has type List, we get a stack overflow during type checking!
assert(not hasSize1(t))