Disabling options when allies are present

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I’m working on a game system where various armies now get a bonus special rule if they don’t take any allies.

I’m having a hard time figuring out how to achieve this. I’ve tried a couple of things, but I haven’t found anything that works yet.

The special rules are presumably best implemented as options, so that's what I've done.

It’s easy to determine that a unit is an ally, using a tag template like [ally.?].

But I need to find a way to get that information to the units/options that are NOT allies. What the best way to do that?

I thought a good way would be to have a script (prelink) to check for [ally.?] and then set a global tag, but scripts can't set global tags, so that doesn't seem to work.