function x_prod(A,B,C)
C  Returns as C the vector cross product of A and B:
C  C = A x B.  Also returns as function value the absolute value of C.
      dimension a(3),b(3),C(3),indx(4)
      data indx/2,3,1,2/
        D = 0.
        do k=1,3
          C(k) = A(indx(k))*B(indx(k+1)) - A(indx(k+1))*B(indx(k))
          D = D + C(k) * C(k)
        enddo
        x_prod = SQRT(D)
        return
      end