code: def find_oxygen_reactions(model):
      
      o2_in_model = helpers.find_met_in_model(model, "MNXM4")
      return set([rxn for met in model.metabolites for
                  rxn in met.reactions if met.formula == "O2" or
                  met in o2_in_model])
docs: Return list of oxygen-producing/-consuming reactions.