updater make_coprime (&$a : NNInt, &$b : NNInt) {
         with function gcd (NNInt <-- $a : NNInt, $b : NNInt) {
             $b = 0 ?? $a !! rtn( a => $b, b => $a mod $b round Down )
         }
         $gcd ::= nlx.lib.gcd( $>a, $>b )
         $a := $a div $gcd round Down
         $b := $b div $gcd round Down
     }