input: 
  def is_subtree(T: Node, S: Node) -> bool:
      """ Check if the binary tree S is a subtree of the binary tree T.
      >>> is_subtree(Node(1), Node(1))
      True
      >>> is_subtree(Node(1), Node(2))
      False
      >>> is_subtree(Node(1, Node(2), Node(3)), Node(1, Node(2), None))
      False
      """
  
output:     if not T and not S:
          return True
      if not T or not S:
          return False
      if T.value == S.value and is_identical(T, S):
          return True
      return is_subtree(T.left, S) or is_subtree(T.right, S)
  
  def is_identical(T: Node, S: Node) -> bool:
      if not T and not S:
          return True
      if not T or not S:
          return False
      return T.value == S.value and is_identical(T.left, S.left) and is_identical(T.right, S.right)