This is a little used facility of the callback mechanisms.  The idea is that you have a value (TValue) you want to change.  Rather than write a callback function for every value you want to change, you make those values TValues.  You then pass the TValue along with the kind of operation you want to do (increment, decrement, etc.).  There is an example of this that changes the number of processors in XWindow.  The hope was that you could reuse the operations on many different TValues, increasing code reuse.  However, in practice many of the callbacks wished to do some kind of error checking and the classes of operator functions would have bollooned.  We started writing more callback functions than TValue/Op functions.

James