anchor.diff:      ! 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
  
positive.code: 
      ! Lengths of lattice vectors.
      ! This defines the cubic simulation cell used in the UEG.
      ! TODO: move to sys_ueg_t.
      real(p), allocatable :: box_length(:) ! ndim.
  
      ! lvecs contains all combinations of the above lattice vectors, where the
  
negative.code:     ! Model (lattice/UEG) Hamiltonians
      ! ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  
      ! See also specific structures.
  
      ! 1, 2 or 3 dimensions.
  