col c1 {
    row {
        box A;
        below box B;
        box C [top=A];
    }
    B->C [via=A@topleft, 
          line.color=red];
    use margin=0;
    text: via A@topleft;
}
below 20 col c2 {
    row {
        box A;
        below box B;
        box C [top=A];
    }
    B->C [via=A@right, 
          line.color=red];
    use margin=0;
    text: via A@right;
}