input: 
  def isSubTree(T, S):
      """Check if the binary tree S is a subtree of the binary tree T.
      
      >>> T = Node(26); T.left = Node(10); T.right = Node(3); T.left.left = Node(4); T.left.right = Node(6); T.right.right = Node(3)
      >>> S = Node(10); S.left = Node(4); S.right = Node(6); S.left.right = Node(30)
      >>> isSubTree(T, S)
      False
      >>> S = Node(10); S.left = Node(4); S.right = Node(6)
      >>> isSubTree(T, S)
      True
      """
      if S is None:
          return True
      if T is None:
          return False
      if areIdentical(T, S):
          return True
      return isSubTree(T.left, S) or isSubTree(T.right, S)
  
  def areIdentical(T, S):
      if T is None and S is None:
          return True
      if T is None or S is None:
          return False
      return (T.data == S.data and areIdentical(T.left, S.left) and
              areIdentical(T.right, S.right))
  
output:     if S is None:
          return True
      if T is None:
          return False
      if areIdentical(T, S):
          return True
      return isSubTree(T.left, S) or isSubTree(T.right, S)
  
  def areIdentical(T, S):
      if T is None and S is None:
          return True
      if T is None or S is None:
          return False
      return (T.data == S.data and areIdentical(T.left, S.left) and
              areIdentical(T.right, S.right))