**Defined in theory: Physical-quantities****Source code: physical-quantities.lisp****Also defined in:**Kif-numbers, Basic-matrix-algebra, Tensor-quantities

**Documentation:**- is the binary subtraction operator for physical-quantities. This is a polymorphic extension of the same function over real numbers as defined in the kif-numbers ontology.

All quantity objects have an additive inverse and the addition of a parameter and its additive-inverse will equal a zero element, such as the real number 0 or the zero vector of the same dimension if ?x is a vector. Each engineering quantity algebra will define specialization of + for its domain wirth a zero element.

(=> (And (Physical-Quantity ?X) (Physical-Quantity ?Y)) (<=> (- ?X ?Y ?Z) (= ?X (+ ?Y ?Z))))