anchor.diff:  program setup_fluid
  +  use common
     use cell_system
     use particle_system
     use hilbert
  
positive.code: 
    type(cell_system_t) :: solvent_cells
    type(particle_system_t) :: solvent
  
    type(h5md_file_t) :: datafile
    type(h5md_element_t) :: elem
  
negative.code: 
    call solvent% sort(solvent_cells)
  
    wall_v = 0
    do i = 1, 200
       call simple_mpcd_step(solvent, solvent_cells, mt)
       call mpcd_stream(solvent, solvent_cells, 0.1d0)
       call solvent% sort(solvent_cells)
       call solvent_cells%count_particles(solvent% pos)
    end do
  
    do i = 1, 200
       call simple_mpcd_step(solvent, solvent_cells, mt)
       v_com = sum(solvent% vel, dim=2) / size(solvent% vel, dim=2)
  
       call mpcd_stream(solvent, solvent_cells, 0.1d0)
  