==== Modification of symbol blocks ====

    def a_blocks (expr p) =
      T:=identity;
      pickup PenC;
      path q, qq; q = bbox p;
      picture tmp_pic; 
      uu := max(u, (xpart urcorner q - xpart llcorner q)/100, (ypart urcorner q - ypart     llcorner q)/100);
      tmp_pic := image(
        for i = xpart llcorner q step 1.0uu until xpart urcorner q:
          for j = ypart llcorner q step 1.0uu until ypart urcorner q:
            qq := punked (((-.5uu,-.5uu)--(.5uu,-.5uu)--(.5uu,.5uu)--(-.5uu,.5uu)--cycle) 
	         randomized (uu/2))
                 rotated uniformdeviate(360) 
                 shifted ((i,j) randomized 1.0uu);
	    if xpart (p intersectiontimes qq) < 0:
	      thclean qq;
	      thdraw qq;
	    fi;
          endfor;  
        endfor;
      );
      clip tmp_pic to p;
      draw tmp_pic;
    enddef;