Clean Elimination Passes

[jehoshua02]

Currently, elimination passes fold into possible as they go, allowing later operations in the pass to be affected by earlier operations in the same pass. I wonder if this is leading to undesired/unreliable/unpredictable side effects.

Just to be safe, and confident, refactor all elimination passes to be clean. In other words, each elimination is wholly dependent on nothing but the state of the puzzle when the pass began, and not affected by any other eliminations within the pass.