NopScript interpreter

def nopscript(prog, nopiness):
    higit = "0123456789abcdefABCDEF"
    i = 0
    stacks = [[], []]
    stacki = 0

    while i < len(prog):
        com = prog[i]
        if com == "$":
            stacks[stacki].pop()

        elif com == "@":
            stacki += 1
            stacki %= 2

        elif com == ".":
            a = stacks[stacki].pop()
            b = stacks[stacki].pop()
            print(chr(int(b+a, 16)))

        elif com in higit:
            stacks[stacki].append(com)

        for x in range(nopiness):
            i += 1
            assert prog[i] == 'x'
        i += 1