==== Modification du symbole vecteur - débris ====

     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;