open TextIO;

     fun put(0) = output(stdOut,"\n")
     |   put(n) = (output(stdOut,"X"); put(n-1));

     fun comb(n,m) = (
             put(n);
             if m=0 orelse m=n then 1
             else comb(n-1,m) + comb(n-1,m-1)
     );

     comb(5,2);