anchor.diff:          ! and p_select = p_gen for normalised probabilities.
           ! p_select is included intrinsically in the algorithm, so we compare
           ! to the probability tau*|H_ij|/p_gen.
  +        ! As we can only excite from (alpha,beta) or (beta,alpha),
  +        !   H_ij =  < ij | ab >  *or*
  +        !        = -< ij | ba >
  +        ! so
  +        !   |H_ij| = U/\Omega
  +        ! for allowed excitations.
           pspawn = tau*hub_k_coulomb/pgen
           psuccess = genrand_real2()
   
  
positive.code: 
          use determinants, only: det_info
          use dSFMT_interface, only:  genrand_real2
          use excitations, only: calc_pgen_hub_k, excit, create_excited_det
          use fciqmc_data, only: tau, spawned_walker_dets, spawned_walker_population
          use system, only: hub_k_coulomb
          use symmetry, only: inv_sym
  
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.
  