Eternity Puzzle - 12 x12

I have made some improvements to my eternity puzzle solver.
It can now solve up to 12x12 within 'reasonable' time, albeit I have to say it is still way too slow for 12x12 (6 minutes). If my calculation is correct, a 12x12 puzzle has:

144! x 4^(144) possible combinations, or roughly
22 x 10^(335) possible combinations.

So, that is not a terrible result. :)
I still have some improvements in mind - although I'm not too sure if they will affect the result that much, since they are merely using language features.

Well.. time will tell. :)

Oh, in case you are wondering how long it takes to solve the 4x4 puzzle with my current solver, it is 4ms.