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four rules now:
1)if a square definately contains a number then the other squares in the same row, column and block cannot be that number
2)if there is only one square in a row, column or block that can be a number then it is that number
3)if there is a square in a row that can only appear in a certain block then remove it as a possibility for the squares in the row that are not in that block. repeat for column/block.
4)if there is a number that can only appear in either of two squares in a row then look to see if there is another number that can also appear in only those two squares. If so then remove the other numbers as possibilities for those two squares. repeat for columns and blocks. repeat for groups of 3,4,5 numbers/squares.
rule 4 took yonks to program correctly, but it's done now. So on to rule 5...
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