anchor.diff:  !> Routines to perform MPCD dynamics
   !!
   !! \manual{algorithms}.
  +!!
  +!!
  +!! MPCD collisions are implemented in simple_mpcd_step and wall_mpcd_step, that takes into
  +!! account a wall in the z-direction.
  +!!
  +!! Streaming is achieved by the mpcd_stream_periodic, mpcd_stream_zwall,
  +!! mpcd_stream_xforce_yzwall and mpcd_stream_nogravity_zwall routines. Only a single stream
  +!! routine should be used, depending on the simulation setup. A further call to md_vel is
  +!! needed in the presence of forces.
   
   module mpcd
     use common
  
positive.code:     end do
  
    end subroutine compute_rho
    
    subroutine compute_vx(particles, vx)
      type(particle_system_t), intent(in) :: particles
      type(profile_t), intent(inout) :: vx
  
negative.code: 
    end subroutine mpcd_stream_xforce_yzwall
  
    !> Collide a particle in y and z with constant acceleration in x
    !!
    !! Periodic boundary conditions are applied in x
    subroutine yzwall_collision(x0, v0, x, v, im, L, t, bc, g)
  