mandelbrot ( re im n -- iter ) //
 [ <reg> ] tri@
 <proto>
 [ 0 <reg> swap [ re<< ] [ im<< ] [ re'<< ] [ im'<< ] quad ]
 ( [ _ const_re<< ] [ _ const_im<< ] [ iter<< dup ] tri* ) fry
 [ ] tri
 ( _ [ re>> ld sq ] [ im>> ld sq add' ] [ 4 ld gt? 1 ld blend iter>> st/sub ] tri
   _ [ re>> ld sq ] [ im>> ld sq sub' ] [ const_re>> add ] [ re'>> st ] quad
   _ [ re>> ld 2 mul ] [ im>> mul ] [ const_im>> add ] [ im'>> st ] quad
   [ im'>> ld im>> st ] [ re'>> ld re>> st ] bi ) fry times