Fixed behavior of within to remove special case.

The second check should be `v > s[len(s)-1]` since otherwise we cannot get the inclusive behaviour I proposed by `s = append(s, s[len(s)-1])` if `s[len(s)-1] != s[len(s)-2]`. It means we may have to search the entire slice, but the functionality is important.

After that LGTM.