box [imargin=0, margin=0,
     line.type=none, fill.color=lgray] {
    box A;
    below rightof box B;
}
box C [middle=A+B, mleft=A+B@mright];
box D [middle=A+B, mright=A+B@mleft];
box E [center=A+B, mtop=A+B@mbottom];
box F [center=A+B, mbottom=A+B@mtop];
