anchor.diff:          !    nattempts: the number of times (on this processor) a random cluster
           !        of excitors is generated in the current timestep.
           !    D0_pos: position in the excip list of the reference.
  -        !    max_cluster_size: the maximum number of excitors that can be
  -        !        included in a single composite cluster of excitors.
  +        !    cumulative_excip_population: running cumulative excip population on
  +        !        all excitors; i.e. cumulative_excip_population(i) = sum(walker_population(1:i)).
  +        !    tot_excip_pop: total excip population.
  +
  +        ! NOTE: cumulative_excip_pop and tot_excip_pop ignore the population on
  +        ! the reference as excips on the reference cannot form a cluster.  Both
  +        ! these quantities should be generated by cumulative_population (or be
  +        ! in the same format).
  +
           ! In/Out:
           !    cdet: information about the cluster of excitors applied to the
           !        reference determinant.  This is a bare det_info variable on input
  
positive.code:                 ! excitors.
                  max_cluster_size = min(min(nel, truncation_level+2), tot_walkers-1)
  
                  ! Allow one spawning & death attempt for each excip on the
                  ! processor.
                  do iattempt = 1, nparticles(1)
  
                      call select_cluster(nparticles(1), D0_pos, max_cluster_size, cdet, cluster)
  
                      if (cluster%excitation_level <= truncation_level+2) then
  
  
negative.code: 
          logical :: soft_exit
  
          integer, allocatable :: cumulative_abs_pops(:)
          integer :: D0_pos, max_cluster_size,tot_abs_pop
          logical :: hit
  
          real :: t1, t2
  