code: def _add_cycle_free(model, fluxes):
      
      model.objective = model.solver.interface.Objective(
          Zero, direction="min", sloppy=True)
      objective_vars = []
      for rxn in model.reactions:
          flux = fluxes[rxn.id]
          if rxn.boundary:
              rxn.bounds = (flux, flux)
              continue
          if flux >= 0:
              rxn.bounds = max(0, rxn.lower_bound), max(flux, rxn.upper_bound)
              objective_vars.append(rxn.forward_variable)
          else:
              rxn.bounds = min(flux, rxn.lower_bound), min(0, rxn.upper_bound)
              objective_vars.append(rxn.reverse_variable)
  
      model.objective.set_linear_coefficients({v: 1.0 for v in objective_vars})
docs: Add constraints for CycleFreeFlux.