def cross(b,c)
        unless b.size == 3 and c.size == 3
          raise "Vectors must be of length 3"
        end
        Vector[b[1]*c[2] - b[2]*c[1], b[2]*c[0] - b[0]*c[2], b[0]*c[1] - b[1]*c[0]]
      end
def cross(b,c)
        unless b.size == 3 and c.size == 3
          raise "Vectors must be of length 3"
        end
        Vector[b[1]*c[2] - b[2]*c[1], b[2]*c[0] - b[0]*c[2], b[0]*c[1] - b[1]*c[0]]
      end
Cross product of two arrays of length 3