# File Geodesic.rb, line 288
        def cross( v2 )
                raise "v1 & v2 only applicable to 3D vectors" unless self.size==3 && v2.size==3
                ax = self[0]; ay = self[1]; az = self[2]
                bx = v2[0];   by = v2[1];   bz = v2[2]
                Vector[
                        ay * bz - by * az,
                        az * bx - bz * ax,
                        ax * by - bx * ay
                ]
        end