define agm  use  $a, $g, $errlim
    # $errlim $g $a "%d %g %d\n" print
    $a 1.0  +   as $t
    repeat $a 1.0 * $g - abs -15 exp10 $a *  >   while
        $a $g + 2 /   as $t
        $a $g * sqrt  as $g
        $t as $a
        $g $a $t  "t: %g a: %g g: %g\n" print
    $a


16   1 2 sqrt /   1   agm   "agm: %.15g\n" print