##Join
##join combine merge
##Using the 'join' keyword you can combine blocks that are side-by-side (with zero margin) to a single visual shape. Each block retains its fill, so the combined block may be filled differently, but you can also override it in the join command.
boxcol a: {
    use color=green;
    box; box; box;
}
boxcol b: {
    use color=blue;
    box; box;
}
box c: [left=a, top=a@bottom] {
    use color = red;
    *oval; *oval;
    *def.actor; *oval;
}
join a+c;