anchor.diff:          ! Note that all the information required for input should be available
           ! during the FCIQMC algorithm and should not be needed to be calculated.
           !
  +        ! Further, we assume only allowed excitations are generated.
  +        !
           ! In:
           !    ij_sym: symmetry spanned by the (i,j) combination of occupied
           !        spin-orbitals from which electrons are excited.
  -        !    ispin: type of excitation. ispin=-1 indicates i,j are both beta
  -        !        orbitals, ispin=0 indicates i,j are an alpha and beta orbital and
  -        !        ispin=1 indicates i,j are both alpha orbitals.
           !    f: bit string representation of the determinant we're exciting
           !        from.
           !    unocc_alpha, unocc_beta: integer list of the unoccupied alpha and
  
positive.code: 
  contains
  
      pure function calc_pgen_hub_k(ij_sym, ispin, f, unocc_alpha, unocc_beta) result(pgen)
  
          ! Calculate the generation probability of a given excitation for the
          ! Hubbard model in momentum space.  The Hubbard model is a special case
  
negative.code:         use symmetry, only: inv_sym, sym_table
  
          real(dp) :: pgen
          integer, intent(in) :: ij_sym
          integer(i0), intent(in) :: f(basis_length)
          integer, intent(in) :: unocc_alpha(nvirt_alpha), unocc_beta(nvirt_beta)
  
  