Convert obj into an Aggregate instance

def mark(obj, map_class=Map, sequence_class=Sequence):
    '''
    Convert obj into an Aggregate instance
    '''
    if isinstance(obj, Aggregate):
        return obj
    if isinstance(obj, dict):
        return map_class(obj)
    if isinstance(obj, (list, tuple, set)):
        return sequence_class(obj)
    else:
        return sequence_class([obj])