anchor.diff:  
     integer :: L(3)
     integer :: i, j, n_neigh
  +  integer :: neighbor_list_size
   
     double precision :: xij(3), dist_sq, cutoff
   
  
positive.code:   test_list = 0
    test_n = 0
  
    write(16,*) neigh% list(:,:)
    write(18,*) neigh% n
  
    do i = 1, 1
       n_neigh = neigh% n(i)
       do j = 1, solvent% Nmax
          xij = rel_pos(colloids% pos(:, i), solvent% pos(:, j), solvent_cells% edges)
          dist_sq = sum(xij**2)
          if (dist_sq < cutoff**2) then
             write(*, '(10f6.2)') colloids% pos(:, i), solvent% pos(:, j), xij, dist_sq
             write(17, *) j
             call test% assert_equal( is_in_list(j, neigh% list(1:n_neigh,i)), .true. )
          end if
       end do
  
negative.code:   call solvent% sort(solvent_cells)
    call solvent_cells% count_particles(solvent% pos)
  
    cutoff = 3.6
    neighbor_list_size = rho * 6 * int(cutoff**3)
  
    call neigh% init(colloids% Nmax, neighbor_list_size)
    allocate(test_n(solvent% Nmax))
    allocate(test_list(300, solvent% Nmax))
  
    call neigh% make_stencil(solvent_cells, cutoff)
  
  