stackoverflow hierarchy question.

interface BinaryTree<TreeType, Node> {
    Node left(TreeType t, Node node);
    Node right(TreeType t, Node node);
}

interface BinaryTreeWithRoot<TreeType, Node> : BinaryTree<TreeType, Node> {
    Node root(TreeType t);
}

interface BinaryTreeWithParentLink<TreeType, Node> : BinaryTree<TreeType, Node> {
    Node parent(TreeType t, Node node);
}

// Now an implementation of the Subtree.

// The subtree object won't be tied to a specific BinaryTree.
class Subtree<Tree, TreeType, Node> : BinaryTreeWithRoot<TreeType, Node>, Tree
    where Tree : BinaryTree<TreeType, Node> {

    // During initialization, initialize the root as a node from the tree.
    public Subtree(Node root);
    Node root;

    override BinaryTreeWithRoot<TreeType, Node>.root(TreeType t) {
        return this.root;
    }
}