Vectors of 3-dimensions. (These vectors have important properities and are of particular interest to engineering analysis).
(<= (Spatial.Dimension $X 3) (3d-Vector-Quantity $X)) (=> (3d-Vector-Quantity ?X) (Spatial.Dimension ?X 3)) (Nth-Domain Cross 3 3d-Vector-Quantity) (Nth-Domain Cross 2 3d-Vector-Quantity) (Nth-Domain Cross 1 3d-Vector-Quantity) (=> (= (Cross ?V1 ?V2) ?V) (3d-Vector-Quantity ?V)) (=> (= (Cross ?V1 ?V2) ?V) (3d-Vector-Quantity ?V2)) (=> (= (Cross ?V1 ?V2) ?V) (3d-Vector-Quantity ?V1))