Return value should be assigned to caller's valOld instance 

    (conventional idiom to work around the fact that Lua can not pass 

    plain data types by reference). 

    The logic of this function is motivated by the need to support "naive" 

    invocations that do not explicitly set both valNew and/or doSet. 

    e.g. local res = self.Atttribute() must deliver the old value. 

    e.g. self.Atttribute(25) must set the value to 25. 

    At the same time, invokers that explicitly set both parameters must 

    also be able to control the attribute. The basic approach is that 

    setting requires a non-nil new value but can be blocked by explicitly 

    using a false doSet.