Grammar rule function callback: **strlen**. This function will measure the
    string length of all subitems of the first item in argument list.

    :param list args: List of function arguments.
    :return: Length of all subitems of the first item in argument list.
    :rtype: int or list

def grfcbk_strlen(args):
    """
    Grammar rule function callback: **strlen**. This function will measure the
    string length of all subitems of the first item in argument list.

    :param list args: List of function arguments.
    :return: Length of all subitems of the first item in argument list.
    :rtype: int or list
    """
    if not args[0]:
        return None
    if isinstance(args[0], list):
        return [len(x) for x in args[0]]
    return len(args[0])