anchor.diff:  
       subroutine choose_ij_mol(occ_list, i, j, ij_sym, ij_spin)
   
  +        ! Randomly select two occupied orbitals in a determinant from which
  +        ! electrons are excited as part of a double excitation.
  +        !
  +        ! In:
  +        !    occ_list: integer list of occupied spin-orbitals in the determinant.
  +        ! Out:
  +        !    i, j: orbitals in determinant from which two electrons are excited.
  +        !        Note that i,j are ordered such that i<j.
  +        !    ij_sym: irreducible representation spanned by the codensity
  +        !        \phi_i*\phi_j.
  +        !    ij_spin: spin label of the combined ij codensity.
  +        !        ij_spin = -2   i,j both down
  +        !                =  0   i up and j down or vice versa
  +        !                =  2   i,j both up
  +
           use basis, only: basis_fns
           use system, only: nel
           use point_group_symmetry, only: cross_product_pg_basis
  
positive.code: 
      pure function calc_pgen_single_mol(occ_list, symunocc, a) result(pgen)
  
          use basis, only: basis_fns
          use fciqmc_data, only: pattempt_single
          use system, only: nel, sym0
  
negative.code: 
          ! In:
          !    f: bit string representation of the Slater determinant from which
          !        an electron is excited.
          !    occ_list: integer list of occupied spin-orbitals in the determinant.
          !    symunocc: number of unoccupied orbitals of each spin and
          !        irreducible representation.  The same indexing scheme as
          !        nbasis_sym_spin (in the point_group_symmetry module) is used.
          ! Out:
          !    i: orbital in determinant from which an electron is excited.
          !    a: previously unoccupied orbital into which an electron is excited.
          !        Not set if allowed_excitation is false.
          !    allowed_excitation: false if there are no possible single
          !        excitations from the determinant which conserve spin and spatial
          !        symmetry.
  
          use basis, only: basis_length, basis_fns, bit_lookup
          use point_group_symmetry, only: nbasis_sym_spin, sym_spin_basis_fns
  