docs: def _prefix_ga(value):
      """Prefix a string with 'ga:' if it is not already
  
      Sort values may be prefixed with '-' to indicate negative sort.
  
      >>> _prefix_ga('foo')
      'ga:foo'
      >>> _prefix_ga('ga:foo')
      'ga:foo'
      >>> _prefix_ga('-foo')
      '-ga:foo'
      >>> _prefix_ga('-ga:foo')
      '-ga:foo'
      """
      prefix = ''
      if value[0] == '-':
          value = value[1:]
          prefix = '-'
      if not value.startswith('ga:'):
          prefix += 'ga:'
      return prefix + value