A0;For the ancient topic of the technicalities of on-way terrain: It should be fully penetrable, including by bombing, but only by lemmings walking or facing (if they're a blocker) in that direction.
After some thought I started to see where you're getting at, but I'm afraid your approach is too simplistic for what you intuitively might have in mind.
I think what you're trying to do is to prevent a situation like this: there's this large rectangular one-way block, say with arrows pointing left. You send a bomber to the left edge of the block. It explodes, and takes out a chunk of the block.
You cried, this isn't right. If I can't bash the block from the left, why should an explosion from the left be able to take the same pixels out?
But here are some problems with using the facing direction as an attempt to fix this issue:
1) okay, so I re-timed my bomber so that he hits the left of the block first, turns around, and then immediately explodes. Now the lemming is facing left, and so by your criteria his explosion will take out a chunk of the block. And yet to the average player that is just bizarre: it's basically the same explosion at the exact same place, involving exactly the same pixels, only that the lemming has turned around.
2) let's say I send a climber up to walk on the top of the block, and make an explosion happen up there. Again, you now have the weird situation that simply due to the facing direction, the same lemming in the same position, with the explosion affecting exactly the same pixels, will have different outcomes.
3) But wait! If we are to be 100% consistent in this, diggers should be affected too. Now things get truly absurd: the digger can only dig when he faces left when up on the block.
You say, no, diggers shouldn't be affected because digging occurs in a vertical direction, not horizontal.
But then what about the explosion at the top of the block? Can you really say that the explosion occurs in any particular direction? The player can also say this: if I bomb at the same horizontal position multiple times, I effectively can bomb my way vertically down the block, which is just like digging vertically down. If digging down does not depend on facing direction, why should bombing down does?
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The real issue here is that, a truly consistent intuitive model of one-way walls in effect requires you to think in terms of forces. Namely that when bashing/mining/digging/bombing removes a particular pixel, it is applying some force in some direction to the pixel in question. For skills like bashing, mining and digging, they are strongly directional (with the direction tied to the lemming's facing direction, for the case of bashers and miners) and so you can assign each pixel the same direction of force, and therefore it's possible to say that either all pixels affected are removed, or none of them are.
But with bombers, the bomber's explosion intuitively is really applying different force vectors to different pixels. In effect you can say the direction of the force is the arrow going from the center of the explosion to the pixel in question. And so for some pixels, the arrow points in one direction, while for others the arrow will point in the totally opposite direction.
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This however does suggest one relatively simple way to handle explosions affecting one-way pixels. Suppose we decided that any force vector who has a component (however small) of direction that is opposite the direction of the one-way arrow, then the pixel affected should not be removed.
What that statement boils down to is then this: when the explosion occurs, the left half of the explosion area cannot take out one-way pixels pointing right, and the right half of the explosion area cannot take out one-way pixels pointing left. This seems to me to be the best way to deal with this technicality without introducing problematic inconsistencies. And it will solve the case of the bomber exploding a chunk of one-way wall from the wrong side, no matter which way the bomber faces when exploding.
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That said, I think we still need to think about what it means to introduce, in affect, a new type of one-way walls, since we already have one type from original Lemmings and another type from Cheapo. Do we really want a third? O_O