docs: def strip_optional_prefix(string, prefix, log=None):
      """
      >>> strip_optional_prefix('abcdef', 'abc')
      'def'
      >>> strip_optional_prefix('abcdef', '123')
      'abcdef'
      >>> strip_optional_prefix('abcdef', '123', PrintingLogger())
      String starts with 'abc', not '123'
      'abcdef'
      """
      if string.startswith(prefix):
          return string[len(prefix):]
      if log:
          log.warn('String starts with %r, not %r', string[:len(prefix)], prefix)
      return string