be careful about optimizing a+b-a-b !

[George Corliss]

Whatever is decided about optimizing a+b-a-b, we must be careful not
to be TOO aggressive.  At least one popular symbolic algebra system
"simplifies"  sin (asin (10)) = 10  !!!  We surely don't want Fortran
or C to do that.

Also, if a and b are user-defined types, such optimization is not
appropriate.  If A is an interval, for example, 
A - A = |A| * [-1, 1] not 0.

    George F. Corliss (corlissamcs.anl.gov)