begin parameters
  R0   100
  L0   500
  RL0   0
  kon  0.01
  koff  0.1
end parameters

begin species
  R   R0    
  L   L0
  RL  RL0
end species

begin reaction rules
   R + L <-> RL kon, koff
end reaction rules

begin observables
  Molecules R_unbound  R
  Molecules L_unbound  L
  Molecules R_complex_L  RL
  Molecules R_total  R RL
  Molecules L_total  L RL
end observables

generate_network();
writeSBML();
simulate_ode({t_end=>50,n_steps=>20});