Perhaps one idea that could be argued for is a rule of "negative overrides positive"; this could be seen by analogy with the existing case of splatting from a long fall overriding exiting.
This has positive and negative consequences: On the one hand, it would allow for nice puzzles ideas (e.g. placing triggered traps above exits), but on the other hand, this might lead to a lot of frustration, if it is not obvious whether a trap trigger reaches over all of the exit or not. Similarly one can mislead players by placing pickup-skills behind continuous traps...
Sure, such designs are certainly to be discouraged, but I worry about the guideline: "Players will certainly not complain if the game is too player-friendly (regarding edge cases), while they will get annoyed if the rules are too unfriendly to users."
Your idea might nevertheless be a good one. I am just saying that I am not yet convinced, that it is the best solution...
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Here another related question: How to prioritize blockers versus force fields?
a) Always prioritize blockers over force fields: Then one might pass through force fields by placing blockers on top of them, which would lead to very weird tricks.
b) Always prioritize force fields over blockers: Then blockers within force fields would have no effect at all and lemmings can just walk through them, which again would lead to very weird tricks.
c) Disallow overlapping blocker fields and force fields: This changes the current behavior, where such overlaps are possible. I am not sure how often one wants to place blockers next to force fields and how big the impact is on existing content.
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On this note - are you working on anything related to the trigger area overlapping (or at least fixing the cancelling bug)?
If the one-way-wall cancelling bug was caused by removing the line
if V in [DOM_STEEL, DOM_ONEWAYLEFT, DOM_ONEWAYRIGHT, DOM_ONEWAYDOWN] then
in TLemmingGame.InitializeObjectMap, then my changes will automatically fix this bug. If TLemmingGame.InitializeObjectMap isn't the cause of the bug, then I have no idea where to find the bug and might only accidentally fix it.