> [I think you mean they're not associative.  I don't know any situations   
   > where a+b != b+a, but lots where a+(b+c) != (a+b)+c -John]   
      
   Take fused multiply-add, and the expression a*b + c*d.   
   It can be evaluated either as fma (a, b, c*d) or fma (c, d, a*b).   
      
   It would seem natural that the code for a*b + c*d and for c*d + a*b   
   would be different, with different results.   
      
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