docs: def flag(name=None):
      """
      Creates the grammar for a Flag (F) field, accepting only 'Y', 'N' or 'U'.
  
      :param name: name for the field
      :return: grammar for the flag field
      """
  
      if name is None:
          name = 'Flag Field'
  
      # Basic field
      field = pp.Regex('[YNU]')
  
      # Name
      field.setName(name)
  
      field.leaveWhitespace()
  
      return field