In Python, How can one subtract two non-unique, unordered lists? Say we have `a = [0,1,2,1,0]` and `b = [0, 1, 1]` I'd like to do something like `c = a - b` and have `c` be `[2, 0]` or `[0, 2]` order doesn't matter to me. This should throw an exception if a does not contain all elements in b.

**Note this is different from sets!** I'm not interested in finding the difference of the sets of elements in a and b, I'm interested in the difference between the actual collections of elements in a and b.

I can do this with a for loop, looking up the first element of b in a and then removing the element from b and from a, etc. But this doesn't appeal to me, it would be very inefficient (order of `O(n^2)` time) while it should be no problem to do this in `O(n log n)` time.