@staticmethod
def group_format(items, converter):
    """
        Return a list of numbered items transformed by a converter
        """
    strings = []
    indexed = []
    index = 1
    for item in items:
        name = converter(item)
        if name is not None:
            indexed.append(item)
            strings.append(f'#{index}: {converter(item)}')
            index += 1
    return indexed, '\n'.join(strings)
