Nim-Value 
Every position of every Impartial Game has a nim-value, making it equivalent to a Nim-Heap. To find the nim-value (also called the Sprague-Grundy Number), take the Mex of the nim-values of the possible moves. The nim-value can also be found by writing the number of counters in each heap in binary, adding without carrying, and replacing the digits with their values mod 2. If the nim-value is 0, the position is Safe; otherwise, it is Unsafe. With two heaps, safe positions are  where . With three heaps, (1, 2, 3), (1, 4, 5), (1, 6, 7), (2, 4, 6), (2, 5, 7), and (3, 4, 7). 

• Mex=minimum excluded，其它的就自己读吧，很简单的英文 (594) frkstyc 2006-07-02 16:10:35