The qualitative proportionality, Qprop+, also known as an indirect influence in the qualitative process theory literature, states that all things being equal ?x is proportional to ?y. If there are no other influences on ?x, then (Qprop+ ?x ?y) is equivalent to (M+ ?y ?x). Otherwise, it means that x = f(...,y,...) and the partial of f w.r.t. y is greater than zero.
(=> (Qprop+ $X $Y) (Time-Dependent-Quantity $Y)) (=> (Qprop+ $X $Y) (Time-Dependent-Quantity $X))