Parses one or more arguments with the installed parser.

    Args:
      arguments: a single argument or a list of arguments (typically a
        list of default values); a single argument is converted
        internally into a list containing one item.

def parse(self, arguments):
    """Parses one or more arguments with the installed parser.

    Args:
      arguments: a single argument or a list of arguments (typically a
        list of default values); a single argument is converted
        internally into a list containing one item.
    """
    new_values = self._parse(arguments)
    if self.present:
      self.value.extend(new_values)
    else:
      self.value = new_values
    self.present += len(new_values)