Returns a restricted <succsOf> which only takes and yields
        values from <l>

def restricted_cover(l, succsOf):
    """ Returns a restricted <succsOf> which only takes and yields
        values from <l> """
    fzl = frozenset(l)
    lut = dict()
    for i in l:
        lut[i] = fzl.intersection(succsOf(i))
    return lambda x: lut[x]