It would be helpful to expand the syntax for including groups to enable subgroups, like this:
command.foo(groups={W1.1, W1.2}-W2-{W3.1,W3.2,W3.3})
command.foo(groups={W1.1,W3.1}-W1.2-W2}
etc.
That way I can rerun the command with a different grouping without having to re-create the .groups file