def __iter__(self):
"""Supports a pre_order traversal on a view of self."""
if not self.is_empty():
stack = LinkedStack()
stack.push(self._root)
while not stack.is_empty():
node = stack.pop()
yield node.data
if node.right is not None:
stack.push(node.right)
if node.left is not None:
stack.push(node.left)
