==== Modification of symbol array - sands ====

    def a_sands (expr p) =
      T:=identity;
      pickup PenC;
      path q; q = bbox p;
      picture tmp_pic;
      tmp_pic := image(
        for i = xpart llcorner q step 0.4u until xpart urcorner q:
          for j = ypart llcorner q step 0.4u until ypart urcorner q:
            draw origin shifted ((i,j) randomized 0.3u) withpen PenC;
          endfor;
        endfor;
      );
      #clip tmp_pic to p;
      draw tmp_pic;
    enddef;
    endcode  

There could be a complication with symbol sands you will not see modified symbol in Acrobat Reader. It is a problem of Acrobat Reader. It is possible for pdfs only change the value for generation of map:

    def a_sands (expr p) =
      T:=identity;
      pickup PenA;
      path q; q = bbox p;
      picture tmp_pic;
      tmp_pic := image(
        for i = xpart llcorner q step 0.4u until xpart urcorner q:
          for j = ypart llcorner q step 0.4u until ypart urcorner q:
            draw origin shifted ((i,j) randomized 0.3u) withpen PenA;
          endfor;
        endfor;
      );
      #clip tmp_pic to p;
      draw tmp_pic;
    enddef;
    endcode  


We changed only the keyword PenC to PenA. You will see the points of sand in Reader now.