use label="";
col x {box a; box b; box c; box d;}
col y {box a; box b: B; box c; box d;}
col v {box a; box b; box c: C; box d;}
col z {box a; box b; box c; box d;}

(x@left, x.b+x.c@middle) ++ (z@right,);
(x@left, x.b+x.c@middle) ++ (z@right,) 
  [color=red, routing=curvy];