anchor.diff:  
     public :: compute_temperature, simple_mpcd_step
     public :: wall_mpcd_step
  -  public :: mpcd_stream
  +  public :: mpcd_stream_periodic, mpcd_stream_zwall
     public :: compute_rho, compute_vx
   
   contains
  
positive.code:        end if
      end do
  
    end subroutine mpcd_stream
  
  end module mpcd
  
negative.code:   !! Advance mpcd particles
    !!
    !! If the cell system has a wall in the z direction, a bounce-back collision is used.
  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
  