def a_debris (expr p) =
   T:=identity;
   % thclean p;
   pickup PenC;
   path q; q = bbox p;
   picture tmp_pic;
   tmp_pic := image(
   for i = xpart llcorner q step 0.7u until xpart urcorner q:
     for j = ypart llcorner q step 0.7u until ypart urcorner q:
      draw ((-.2u,0)--(.2u,0))
      rotated uniformdeviate(360)
      shifted ((i,j) randomized 0.4u) withpen PenC;
     endfor;
   endfor;
  );
  clip tmp_pic to p;
  draw tmp_pic;
  enddef;