anchor.diff:    !! Perform second velocity-Verlet step for quaternion dynamics
     !!
     !! \manual{algorithms/quaternions}
  -  subroutine rigid_body_vv2(this, ps, dt, edges)
  +  subroutine rigid_body_vv2(this, ps, dt)
       class(rigid_body_t), intent(inout) :: this
       type(particle_system_t), intent(inout) :: ps
       double precision, intent(in) :: dt
  -    double precision, intent(in) :: edges(3)
   
       double precision :: L_body(3), omega(3), omega_body(3)
  +    double precision :: pos(3)
       integer :: i
   
       this%L = this%L + this%torque*dt/2
  
positive.code:     this%vel = this%vel + this%force*dt/(2*this%mass)
  
      do i = this%i_start, this%i_stop
         ps%vel(:,i) = this%vel + cross(omega, ps%pos(:,i)+ps%image(:,i)*edges-this%pos)
      end do
  
    end subroutine rigid_body_vv2
  
negative.code:   end subroutine rigid_body_init
  
    !! Compute torque and force on the rigid body in the lab frame
    subroutine rigid_body_compute_force_torque(this, ps)
      class(rigid_body_t), intent(inout) :: this
      type(particle_system_t), intent(in) :: ps
  
      double precision :: pos(3), f(3)
      integer :: i
  
      this%force = 0
  