anchor.diff:    !! Advance mpcd particles
     !!
     !! If the cell system has a wall in the z direction, a bounce-back collision is used.
  -subroutine mpcd_stream(particles, cells, dt,g)
  +subroutine mpcd_stream_zwall(particles, cells, dt,g)
       type(particle_system_t), intent(inout) :: particles
       type(cell_system_t), intent(in) :: cells
       double precision, intent(in) :: dt
  
positive.code:        end if
      end do
  
    end subroutine mpcd_stream
  
  end module mpcd
  
negative.code: 
    public :: compute_temperature, simple_mpcd_step
    public :: wall_mpcd_step
    public :: mpcd_stream_periodic, mpcd_stream_zwall
    public :: compute_rho, compute_vx
  
  contains
  