code: def prune_unused_reactions(cobra_model):
      
  
      output_model = cobra_model.copy()
      reactions_to_prune = [r for r in output_model.reactions
                            if len(r.metabolites) == 0]
      output_model.remove_reactions(reactions_to_prune)
      return output_model, reactions_to_prune
docs: Remove reactions with no assigned metabolites, returns pruned model
  
      Parameters
      ----------
      cobra_model: class:`~cobra.core.Model.Model` object
          the model to remove unused reactions from
  
      Returns
      -------
      output_model: class:`~cobra.core.Model.Model` object
          input model with unused reactions removed
      reactions_to_prune: list of class:`~cobra.core.reaction.Reaction`
          list of reactions that were removed