anchor.diff:              ! label an excitor), the probability of selecting a given cluster is
               ! proportional to the number of ways the cluster could have been
               ! formed.  (One can view this factorial contribution as the
  -            ! factorial prefactors in the series expansion of e^T.)
  +            ! factorial prefactors in the series expansion of e^T---see Eq (8)
  +            ! in the module-level comments.)
               cluster%pselect = cluster%pselect*factorial(cluster%nexcitors)
   
               if (cluster_population /= 0) cluster%excitation_level = get_excitation_level(f0, cdet%f)
  
positive.code: !    set of annihilation and creation operators.  Each permutation causes a sign
  !    change and thus an overall odd number of permutations causes a sign change
  !    in the excip population.  See collapse_cluster.
  ! #. Applying the i-th excitor to D_0 can result in a sign change due to the
  !    different definitions of an excitor and determinant which also requires
  !    permutations of creation and annihilation operators.  See
  !    convert_excitor_to_determinant.
  ! #. As we are creating an excip on the j-th excitor from the i-th excitor, we
  !    must be consistent with our definition of the excitor; in particular they
  !    might produce determinants of different signs when applied to the reference
  !    determinant.  Thus if we consider creating an excip on t_j from t_i (where
  !    j can be i or any connected excitor), we *must* take into account the signs
  !    of the excitors relative to the reference determinant.
  !
  ! In order for the correct dynamics to be performed, we must carefully
  
negative.code: ! hard to maintain in CCMC.  Instead we follow the prescription of Alex Thom and
  ! introduce an additional variable, N_0, to act as a normalisation constant::
  !
  !     |\Psi_{CC}> = N_0 e^{T/N_0} | D_0 >    (11)
  !
  ! The number of particles on the reference (not strictly excips though we shall
  ! refer to them by this name for the sake of conciseness and generality) is
  