docs: def get_attribute_references(instring):
      """
      Return a list of attribute references in the condition expression.
  
      attribute_reference ::= relation_name "." attribute_name | attribute_name
  
      :param instring: a condition expression.
      :return: a list of attribute references.
      """
      parsed = ConditionGrammar().conditions.parseString(instring)
      result = parsed if isinstance(parsed[0], str) else parsed[0]
      return result.attribute_reference.asList() if result.attribute_reference else []