old_contents: class Node(object):
      node.left
      key
      value
      node.right
  
new_contents: class Leaf(object):
      '''Create leaf.'''
      def __init__(self, left=None, key=None, value=None, right=None):
          self.left = left
          self.key = key
          self.value = value
          self.right = right
  
subject: create leaf class.