%% syntaxe : x0 epsilon {f'} {f} newton_solve
/newton_solve {
5 dict begin
   /f exch def
   /f' exch def
   /epsilon exch def
   /x0 exch def
   /x1 0 def
   {
      /x1 x0 dup f x0 f' div sub store
      x1 x0 sub abs epsilon le 
   	 {exit}
   	 {/x0 x1 store}
      ifelse
   } loop
   x1
end
} def