Automatic redefining

[Hassediagram]

Dear Python-forum members,

I have registered to this forum because i have a small but resistant problem.
It is surely trivial, but i have not practiced Python for a long time and i don't remember the trick, and i would be grateful to benefit from someone's help.

(This def is to produce neighbors in a Hasse diagram).

Let "Collection" be a collection of vectors, initially empty.

From an initial vector that i call : "SET", i will produce other vectors that i will add to "Collection". Then i return "Collection".

To do this, I will add or remove elements of SET, and i add these vectors to "Collection".

My problem is that i don't want "SET" to change. When i write C=SET, i want to modify C, but i don't want "SET" to be modified when i modify C. However with this code, "SET" is modified when i modify C. (I can see the change with : print("SET before",SET); and : print("SET after",SET))

def PHa(SET):
    Collection=[]
    for k in range(1,n+1):
        if k not in SET:
            C=SET
            print("SET before",SET)
            C.append(k)
            print("SET after",SET)
            Collection.append(C)
    for k in range(1,n+1):
        if k in SET:
            C=SET
            C.remove(k)
            Collection.append(C)
    return Collection

Does someone know the trick to fix "SET" ?

Thank you very much in advance,
Hassediagram