(* These are alternative solutions to problem 4 (by two 330 students)
 * Thanks to Matthew Barr and Rob Kiefer!  
 *)

(* by Matthew Barr *)
let rec unzip1 l = match l with
  [] -> ([],[])
| (h1,h2)::t -> match (unzip1 t) with
                (t1,t2) -> (h1::t1, h2::t2);; 

(* by Rob Kiefer *)
let rec unzip2 l = match l with
[] -> ([],[])
| ((a,b)::t) -> let x = unzip2 t in
let split (c,d) (x,y) = (c::x, d::y)
in split (a,b) (x);;