code: def idem(cls, ops, kwargs):
      
      return sorted(set(ops), key=cls.order_key), kwargs
docs: Remove duplicate arguments and order them via the cls's order_key key
      object/function.
      E.g.::
  
          >>> class Set(Operation):
          ...     order_key = lambda val: val
          ...     simplifications = [idem, ]
          >>> Set.create(1,2,3,1,3)
          Set(1, 2, 3)