code: def plugin(tree, file_tokens):
      
      for token in file_tokens:
          if token[0] != STRING:  # token[0] == token.type
              continue
          # token[1] == token.string # python 3
          invalid_sequence_match = invalid_escape_sequence_match(token[1])
          if invalid_sequence_match:
              yield (
                  token[2][0],  # line_number
                  token[2][1],  # offset
                  'IES: invalid escape sequence %s' %
                  invalid_sequence_match.group(1),  # text
                  None  # check # unused
              )
docs: Walk the tree and detect invalid escape sequences.