<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def Expand(self, macro_ref_str):
 """Expands the macro reference. Args: macro_ref_str: String of a macro reference (i.e. foo(a, b)). Returns: The text from the expansion. Raises: PDDMError if there are any issues. """

    match = _MACRO_RE.match(macro_ref_str)
    if match is None or match.group(0) != macro_ref_str:
      raise PDDMError('Failed to parse macro reference: "%s"' % macro_ref_str)
    if match.group('name') not in self._macros:
      raise PDDMError('No macro named "%s".' % match.group('name'))
    return self._Expand(match, [], macro_ref_str)