[boost] [qvm] deduce_xx traits wouldn't introduce ODR issues.

Hi, I wonder if the deduce_xx traits wouldn't introduce ODR issues. Remember the TBoost.Conversion library where conversion between two types was customizable. The fact that deduce_xx traits have two types would promote ODR as there is no one type that is expected to do the customization. I believe that changing the default to something depending on the first argument will be better than using the specific qvm type as result. If this is not enough, you could add symbolic operations taken the result type as parameter, e.g. if a+b is not something specified by A, the user could use sum<C>(a,b) Forget my comment if there is no ODR issue. Best, Vicente