Collatz...

fun col n x =
   if x = 1 then (x+1)
   else if (n mod 2) = 0 then (col(n div 2) x+1)
   else (col(3*n+1) x+1);

fun collatz x = col 0;

fun max a b =
   if a > b then a
   else b;

fun tnp a b c =
   if a = b then c
   else tnp((a+1) b (max(collatz a) c));

fun tnp1 a b = tnp a b 0;