anchor.diff:    pos => pos1
     pos_old => pos2
   
  -  L = [8, 5, 5]
  +  L = [120, 30, 30]
   
     call random_number(pos)
     do i = 1, 3
  
positive.code:   type(cell_system_t) :: solvent_cells
    type(tester_t) :: tester
  
    integer, parameter :: N = 2000
    double precision, target :: pos1(3, N), pos2(3, N)
    double precision, pointer :: pos(:,:), pos_old(:,:), pos_pointer(:,:)
  
    integer :: i, L(3), seed_size, clock
    integer, allocatable :: seed(:)
    integer :: h, c1(3), c2(3)
  
    call tester% init()
  
  
negative.code: 
    call solvent_cells%count_particles(pos)
  
    call solvent_cells%sort_particles(pos, pos_old)
  
    pos_pointer => pos
    pos => pos_old
    pos_old => pos_pointer
  
    call solvent_cells%count_particles(pos)
  
    call tester% assert_equal(solvent_cells%cell_start(1), 1)
  