def gen_strings(letters, len, acc):
        if len == 0:
                yield acc
                return

        for letter in letters:
                for perm in gen_strings(letters, len-1, acc+letter):
                        yield perm

def gen_perms(letters, max_len):
        perms = []
        for i in range(1, max_len+1):
                for perm in gen_strings(letters, i, ''):
                        perms.append(perm)
        return perms