'Apply a function of two arguments cumulatively to the items of
a sequence, from right to left, so as to reduce the sequence to
a single value.'

def test_foldr(self):

    self.assertEqual(irc._foldr(operator.sub, 0, [1, 2, 3, 4]), (-2))
    def insertTop(l, x):
        l.insert(0, x)
        return l
    self.assertEqual(irc._foldr(insertTop, [], [[1], [2], [3], [4]]), [[[[[], 4], 3], 2], 1])
