Defines the specialization of the QUANTITY addition operator + for unary-scalar-function-quantitys. This now works when the exact-domains of the addends are overlapping.
(=> (And (Unary-Scalar-Function-Quantity ?X) (Unary-Scalar-Function-Quantity ?Y) (+ ?X ?Y ?Z)) (Unary-Scalar-Function-Quantity ?Z))