(*P26 (**) Generate the combinations of K distinct objects chosen from the N
elements of a list

In how many ways can a committee of 3 be chosen from a group of 12 people? We
all know that there are C(12,3) = 220 possibilities (C(N,K) denotes the
well-known binomial coefficients). For pure mathematicians, this result may be
great. But we want to really generate all the possibilities in a list.

Example:
* (combination 3 '(a b c d e f))
((A B C) (A B D) (A B E) ... )*)

let comb n l =
    let rec comb l c =
        if( List.length c = n) then [c] else
        match l with
        [] -> []
        | (h::t) -> (comb t (h::c))@(comb t c)
    in
        comb l []
;;

List.iter
    (fun c -> (
        (List.iter (Printf.printf "%d ") c);
        Printf.printf "\n"))
    (comb 3 [1;2;3;4;5;6])
;;