code: def index(self, item):
          
          for count, other in enumerate(self):
              if item == other:
                  return count
          raise ValueError('%r is not in OrderedSet' % (item,))
docs: Find the index of `item` in the OrderedSet
  
          Example:
              >>> # ENABLE_DOCTEST
              >>> import utool as ut
              >>> self = ut.oset([1, 2, 3])
              >>> assert self.index(1) == 0
              >>> assert self.index(2) == 1
              >>> assert self.index(3) == 2
              >>> ut.assert_raises(ValueError, self.index, 4)