code: def subtract_metabolites(self, metabolites, combine=True, reversibly=True):
          
          self.add_metabolites({
              k: -v for k, v in iteritems(metabolites)},
              combine=combine, reversibly=reversibly)
docs: Subtract metabolites from a reaction.
  
          That means add the metabolites with -1*coefficient. If the final
          coefficient for a metabolite is 0 then the metabolite is removed from
          the reaction.
  
          Notes
          -----
          * A final coefficient < 0 implies a reactant.
          * The change is reverted upon exit when using the model as a context.
  
          Parameters
          ----------
          metabolites : dict
              Dictionary where the keys are of class Metabolite and the values
              are the coefficients. These metabolites will be added to the
              reaction.
  
          combine : bool
              Describes behavior a metabolite already exists in the reaction.
              True causes the coefficients to be added.
              False causes the coefficient to be replaced.
  
          reversibly : bool
              Whether to add the change to the context to make the change
              reversibly or not (primarily intended for internal use).