code: def lookup(cls, backend, obj):
          
          ids = set([el for el in obj.traverse(lambda x: x.id) if el is not None])
          if len(ids) == 0:
              raise Exception("Object does not own a custom options tree")
          elif len(ids) != 1:
              idlist = ",".join([str(el) for el in sorted(ids)])
              raise Exception("Object contains elements combined across "
                              "multiple custom trees (ids %s)" % idlist)
          return cls._custom_options[backend][list(ids)[0]]
docs: Given an object, lookup the corresponding customized option
          tree if a single custom tree is applicable.