anchor.diff:  
           ! In:
           !    sys: system object being studied.
  -        !    ij_sym: irreducible representation spanned by the (i,j) codensity.
  +        !    ij_sym: irreducible representation spanned by the <i,j| codensity.
           !        As symmetry is conserved in allowed excitations, this is also
           !        the irreducible representation spanned by the (a, b) codensity.
           !    a, b: unoccupied orbitals into which electrons are excited.
  
positive.code:         !    sys: system object being studied.
          !    f: bit string representation of the Slater determinant from which
          !        an electron is excited.
          !    sym: irreducible representation spanned by the (i,j) codensity.
          !    spin: spin label of the selected (i,j) pair.  Set to -2 if both ia
          !        and j are down, +2 if both are up and 0 otherwise.
          !    symunocc: number of unoccupied orbitals of each spin and
  
negative.code:                 ! Conserve symmetry (spatial and spin) in selecting a.
                  ims = (sys%basis%basis_fns(i)%Ms+3)/2
                  ! Assume op_sym is self-conjugate.
                  ! We want
                  !   Gamma_totsym = Gamma_<D|o|D_i^a>
                  !                = Gamma_D* Gamma_op Gamma_i^-1 Gamma_a Gamma_D
                  !                = Gamma_i^-1 Gamma_op Gamma_a
                  ! Thus we require
                  !   Gamma_a = Gamma_i Gamma_op
                  isym = sys%read_in%cross_product_sym_ptr(sys%read_in, sys%basis%basis_fns(i)%sym, op_sym)
                  if (symunocc(ims, isym) /= 0) then
                      ! Found i.  Now find a...
  