def filter(self):
l = LazyList(range(30))
l2 = l.filter(_ % 2 == 0)
l2.strict.should.have.length_of(0)
l3 = LazyList(range(30))
l3[29]
l4 = l3.filter(_ % 2 == 0)
l4.strict.should.have.length_of(15)
l4.drain.should.equal(List.wrap(range(0, 30, 2)))
