<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def append(self, node):
 """Append a statement to the current statement. Note that multiple calls to append will result in the last statement to be appended to end up at the bottom. Args: node: The statement to append. Raises: ValueError: If the given node is not a statement. """

    if not isinstance(node, grammar.STATEMENTS):
      raise ValueError
    self.to_append[-1].append(node)