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))