lhs = mod[0] -> tee();
rhs = mod[1] -> tee();

lhs -> [0]joined;
rhs -> [1]joined;

joined = join() -> map(|(k, (lhs, rhs))| (k, (lhs, Some(rhs)))) -> combined;

lhs -> [pos]missed;
rhs -> map(|(k, _v)| k) -> [neg]missed;

missed = anti_join()
    -> map(|(k, v)| (k, (v, None)))
    -> combined;

combined = union() -> mod;