Both IDs are kept, with the left sides first.
Both IDs are kept, with the left sides first. The left IDs remain in order.
Right IDs are discarded, left IDs are kept, in order.
All IDs are kept.
All IDs are kept. Prefix first, then remaining left IDs, then remaining right IDs. The result is in order of the prefix/key.
This should also be used in Morph1 to indicate the IDs are retained.
TODO: Much like join, custom Morph2's should be allowed to specify order after the join.
Left IDs are discarded, right IDs are kept, in order.