The C+ relation between two quantities x and y means that x = f(...,y,...) and the partial of f w.r.t. y is 1. If all of the arguments to f are C+, this is equivalent to saying that x is their sum.
(=> (C+ $X $Y) (Time-Dependent-Quantity $Y)) (=> (C+ $X $Y) (Time-Dependent-Quantity $X))