a 2-dimensional stream
cross sectional area of the stream
(Forall (?Self ?Time)
(<=> (Active ?Time 2d-Stream ?Self)
(And (2d-Stream ?Self)
(Active ?Time Stream ?Self)
(Active ?Time Terminal ?Self))))
(=> (2d-Stream ?Self)
(Quantity.Dimension (Stream-Area ?Self) Area-Dimension))