When writing tests, try to write the following cases:

1. common use
2. zero-element cases (empty list, int 0, etc.)
3. exception triggers (wrap like optNF last [] = None, use specific exceptions)
4. negative-element cases (e.g. negative indices)
5. one-element cases ([1], "a", 1, ...)

6. if the domain of the function is small, how about an exhaustive check?
   e.g. smap Char.uppercase ('\x00'--^'\xff') = uppercase ('\000'--^'\255')

7. in-order checks for iterators where possible and sensible
   e.g. (let i = ref 0 in ignore (map (fun j -> i:=j;j) (1--10)); !i = 10)