Make set.union and set.intersection convert a non-set self implicitly

[tjreedy (Terry Jan Reedy)]

I cannot think of any precedent for what you are requesting. It violates the basic idea of an instance method. Anyway, give a list of lists:

lists = [[1, 2, 3], [2, 3, 4], [3, 4, 5]]
set(lists.pop()).intersection(*lists)
# {3}