(<=> (3d-Point ?Pt) (Point-In ?Pt 3d-Length-Space))
(<= (Point-In $X 3d-Length-Space) (3d-Point $X)) (<=> (3d-Point ?Pt) (Point-In ?Pt 3d-Length-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))))))) (Nth-Domain Position 2 3d-Point) (Nth-Domain Position 1 3d-Point)