def as_list(c):
    """Ensures that the result is a list.

    If input is a list/tuple/set, return it.
    If it's None, return empty list.
    Else, return a list with input as the only element.
    
    """

    if isinstance(c, (list,tuple,set)):
        return c
    elif c is None:
        return []
    else:
        return [c]