anchor.diff:  
     g = 0
     g(1) = PTread_d(config, 'g', loc=params_group)
  -  bufferlength = PTread_i(config, 'buffer_length', loc=params_group)
  +  buffer_length = PTread_i(config, 'buffer_length', loc=params_group)
     max_speed = PTread_d(config,'max_speed', loc=params_group)
     prob = PTread_d(config,'probability', loc=params_group)
     alpha = PTread_d(config,'alpha', loc=params_group)
  
positive.code:   double precision :: g(3) !gravity
    logical :: fixed, on_track, stopped, N_in_front, dimer, N_type
    logical :: store_rho_xy
    integer :: bufferlength
    double precision :: max_speed, z, Lz
    integer :: steps_fixed
    type(args_t) :: args
  
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)
  