subroutine spinorextend(za,zb)
      implicit none
C-----Authors: John Campbell and Keith Ellis, November 2011
C-----given spinor products calculated for heavy quark auxiliary vectors
C-----eta_1(e1) and eta_4(e4), return spinor products extended to yield also products
c-----for the opposite polarization of the auxiliary vector.
      include 'constants.f'
      include 'zprods_decl.f'
      include 'masses.f'
      include 'etadef.f'
      integer j

      do j=1,6
      za(e1p,j)=zb(e1,1)*za(1,j)/mt 
      za(j,e4p)=-za(j,4)*zb(4,e4)/mt 
      za(j,e1p)=-za(e1p,j)
      za(e4p,j)=-za(j,e4p)

c--- use c.c. to obtain zb from za      
      zb(j,e1p)=za(e1,1)*zb(1,j)/mt 
      zb(e4p,j)=-zb(j,4)*za(4,e4)/mt
      zb(e1p,j)=-zb(j,e1p)
      zb(j,e4p)=-zb(e4p,j)
      enddo
      za(e1p,e4p)=-zb(e1,1)*za(1,4)*zb(4,e4)/mt**2
      za(e4p,e1p)=-za(e1p,e4p)
c--- use c.c. to obtain zb from za      
      zb(e4p,e1p)=+za(e1,1)*zb(1,4)*za(4,e4)/mt**2
      zb(e1p,e4p)=-zb(e4p,e1p)
      return
      end