docs: def strip_required_prefix(string, prefix):
      """
      >>> strip_required_prefix('abcdef', 'abc')
      'def'
      >>> strip_required_prefix('abcdef', '123')
      Traceback (most recent call last):
      ...
      AssertionError: String starts with 'abc', not '123'
      """
      if string.startswith(prefix):
          return string[len(prefix):]
      raise AssertionError('String starts with %r, not %r' % (string[:len(prefix)], prefix))