The wiki (thanks, Eric) informs us about the incompatible_with_equipment-key:
What I've found out by testing is that, if I use a list of items, they are combined with an AND.requires_equipment and incompatible_with_equipment can be a single item or a space separated list of items
So in case of the requires_equipment the item in whose equipment.plist-entry the requires_equipment-key is found will only be available if all the items from the list are already installed. (And this is what most people would expect, I guess.)
But the same is true in case of the incompatible_with_equipment-key. The item in whose equipment.plist-entry the incompatible_with_equipment-key is found will only be not available if all the items from the list are already installed. So it is incompatible only with all the specified items together.
And this was not what I was expecting in the first place (although it is logical once you think about it). I was expecting an OR-query, so that the equipment I was scripting would be incompatible with any of the items specified in the list, meaning it would not be available if any one item in the list were installed.
In my case I could get around this by a script, but I think all scripters should be aware that the incompatible_with_equipment-key, if used with a list of items, may work counter-intuitive (at least if their intuition is like mine), and the wiki should be amended to explain that.
