function uid(alphabet, n) {
???
}
uid("ab", 1) // One of: "a", "b"
uid("ab", 2) // One of: "aa", "ab", "ba", "bb"
uid("ab", 3) // One of: "aaa", "aab", "aba" ...
uid("abc", 1) // One of: "a", "b", "c"
uid("abc", 2) // One of: "aa", "ab", "ac", "ba", "bb", "bc", "ca", "cb", "cc"
uid("abc", 3) // One of: "aaa", "aab", "aac" ...