anchor.diff:          type(det_info), intent(in) :: cdet
   
           real(dp) :: pgen, psuccess, pspawn
  -        integer :: i, j, a, b, ij_sym, ab_sym, nparticles
  +        integer :: i, j, a, b, ij_sym, ab_sym, nparticles, s
           integer(i0) :: f_new(basis_length)
           type(excit) :: connection
   
  
positive.code:             ! on...
              call choose_ab_hub_k(cdet%f, cdet%unocc_list_alpha, cdet%unocc_list_beta, ab_sym, a, b)
  
              ! 5. Is connecting matrix element positive (in which case we spawn with
              ! negative walkers) or negative (in which case we spawn with positive
              ! walkers)?
              if (mod(i-a,2) == 0) then
                  ! i,a are both alpha spin-orbitals or both beta spin-orbitals.
                  ! Thus the connecting matrix element comes from the Coulomb
                  ! integral and is positive.
                  nparticles = -nparticles
              end if
  
              ! 6. Set info in spawning array.
              connection%nexcit = 2
              connection%from_orb = (/ i,j /)
              connection%to_orb = (/ a,b /)
              call create_excited_det(cdet%f, connection, f_new)
              spawned_walker_dets(:,spos) = f_new
              spawned_walker_population(spos) = nparticles
  
negative.code:         ! See choose_ij_hub_k for a specific procedure for the momentum space
          ! formulation of the hubbard model.
          ! In:
          !    occ_list: integer list of occupied spin-orbitals in a determinant.
          ! Out:
          !    i, j: randomly selected spin-orbitals.
          !    ij_sym: symmetry label of the (i,j) combination.
  