The default code for symbol area ice is this one :

  def a_ice (expr p) =
    T:=identity;
    pickup PenC;
    path q; q = bbox p;
    picture tmp_pic, tmp_ice; 
    pair z;
    tmp_ice := image (
      draw (-.2u,0)--(.2u,0);
      draw (0,-.2u)--(0,.2u);
    );
    tmp_pic := image(
    for i = xpart llcorner q step 1.5u until xpart urcorner q:
      for j = ypart llcorner q step 1.5u until ypart urcorner q:
        z := ((i,j) randomized 1.3u);
        if xpart (p intersectiontimes ((bbox tmp_ice) shifted z)) < 0:
          draw tmp_ice shifted z;
        fi;
        endfor;  
      endfor;
    );
    clip tmp_pic to p;
    draw tmp_pic;
  enddef;