The distance function specialized to points in 3D space.
(=> (And (3d-Point ?P1) (3d-Point ?P2)) (<=> (Distance ?P1 ?P2 ?D) (And (Scalar-Quantity ?D) (Quantity.Dimension ?D Length-Dimension) (= ?D (Sqrt (Dot (Position ?P1 ?P2) (Position ?P1 ?P2)))))))