Difference method not working for certain cases

[muety]

difference() method only works for a pair of tiles where one is top-right compared to the other. If they are arranged in the opposite way, namely that the second is top-left of the first, an empty list is returned.

Example

# Correct
qk1 = quadkey.QuadKey('021')
qk2 = quadkey.QuadKey('011')
qk1.difference(qk2) # ['011', '013', '031', '010', '012', '030', '001', '003', '021']

# Not correct
qk1 = quadkey.QuadKey('033')
qk2 = quadkey.QuadKey('003')
qk1.difference(qk2) # []