anchor.diff:      ! Model (lattice/UEG) Hamiltonians
       ! ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   
  -    ! TODO: logical ordering, commenting.
  -
       ! See also specific structures.
   
       ! 1, 2 or 3 dimensions.
  
positive.code:     ! Contains integer lattice lengths. If less than 3 dimensions are used
      ! then the corresponding unused components are set to 1.
      ! This is useful for making loops over all dimension general.
      ! TODO: move to determinants (ie the sole place where it is actually used).
      integer :: lattice_size(3)
  
  end type sys_lattice_t
  
negative.code: 
      ! Lengths of lattice vectors.
      ! This defines the cubic simulation cell used in the UEG.
      real(p), allocatable :: box_length(:) ! ndim.
  
      ! lvecs contains all combinations of the above lattice vectors, where the
  