This is difficult because contents is a special list and its format is very particular. For one thing, objs will always precede mobs in the list, so at best you could move objs and mobs around relative to each other. But both are also stored as linked lists, so changing around their order is a bit complicated. It's probably easier to copy the list, do a swap, then assign it again.

I never made a push on improving the ability to use Swap() here because the use cases are almost nonexistent.