anchor.diff:    double precision :: g(3) !gravity
     logical :: fixed, on_track, stopped, N_in_front, dimer, N_type
     logical :: store_rho_xy
  -  integer :: bufferlength
  +  integer :: buffer_length
     double precision :: max_speed, z, Lz
     integer :: steps_fixed
     type(args_t) :: args
  
positive.code: 
          if (on_track) then
             if (dimer) then
                if ((colloids% pos(1,1) > (bufferlength+sigma_C)) &
                .and. (colloids% pos(1,2) > (bufferlength+sigma_N))) then
                   on_track = .false.
                   write(*,*) 'on_track', on_track
                end if
             else
                if (colloids% pos(1,1) > (bufferlength+sigma_sphere)) then
                   on_track = .false.
                   write(*,*) 'on_track', on_track
                end if
  
negative.code:      N_colloids = 1
    end if
    L = PTread_ivec(config, 'L', 3, loc=params_group)
    L(1) = L(1) + buffer_length
  
    rho = PTread_i(config, 'rho', loc=params_group)
    N = rho *L(1)*L(2)*L(3)
  