exception BadN;

     exception BadM;

     fun comb(n,m) =
             if n<0 then raise BadN
             else if m<0 orelse m>n then raise BadM
             else if m=0 orelse m=n then 1
             else comb(n-1,m) + comb(n-1,m-1);

     comb(5,2);

     comb(~1,0);

     comb(5,6);