The collections described in Chapter 3 were all linear, meaning they could all be represented by a linearly written list. A common nonlinear data structure in computer science is the binary tree. Implement the class BinaryTree. Instances of BinaryTree contain a left subtree (or nil), a right subtree (or nil), and a value. Instances of the class should respond to the following messages:
left Return the left subtree, usually either nil or other instances of BinaryTree.
left: Set the left subtree to the argument value.
right Return the right subtree.
right: Set the right subtree to the argument value.
value Return the value of the current node.
value: Set the value of the current node to the argument.
How should instances of BinaryTree respond to the enumeration messages first and next? How about print or printString? What should be the superclass of BinaryTree?