% Adds a reference grid, sized to the contents of currentpicture
% useful for marking up external images etc
begingroup;
 save llx, lly, urx, ury, u; u = 10;
 (llx, lly) = llcorner currentpicture;
 (urx, ury) = urcorner currentpicture;
 drawoptions(withpen pencircle scaled 1/4);
  for x = ceiling (llx / u) upto floor (urx / u):
   draw (x*u, lly) -- (x*u, ury) withcolor if x mod u = 0: red else: 1/2 fi;
  endfor
  for y = ceiling (lly / u) upto floor (ury / u):
   draw (llx, y*u) -- (urx, y*u) withcolor if y mod u = 0: red else: 1/2 fi;
  endfor
 drawoptions();
endgroup;