code: def find_nonzero_constrained_reactions(model):
      
      lower_bound, upper_bound = helpers.find_bounds(model)
      return [rxn for rxn in model.reactions if
              0 > rxn.lower_bound > lower_bound or
              0 < rxn.upper_bound < upper_bound]
docs: Return list of reactions with non-zero, non-maximal bounds.