code: def rsub(self, items):
          
          ignore = self._seen
          seen = set()
          add = seen.add
          items = [i for i in items
                   if i not in ignore and i not in seen and not add(i)]
          return self._fromargs(seen, items)
docs: Return order preserving unique items not in this collection.
  
          >>> Unique(['spam']).rsub(['ham', 'spam', 'eggs'])
          Unique(['ham', 'eggs'])