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;
	}
}