有个 Python 问题：从字符串 "aa" 至 "mm" 的排列组合打印出来，例如 "aa" "ab" "ac"... "km" "lm" "mm"，代码应该怎样写？

import string
a2m = string.lowercase[:13]
print ["%s%s" % (x, y) for x in a2m for y in a2m]