Usage as a backend for a SET-like datastructure

I want to use your implementation for my data structure.
In particular set would behave like this:
```
s = set()
s.add(1)
s.add(2)

# Logarithmic search
one_present = 1 in s

# Only one 2 is present
s.add(2)
```

As you might notice, in this case, the insertion of an already existing Node would just override the existing one, not insert it somewhere else.
Do you think this is a useful addition and should I create a PR with that patch, or you're fine with the current implementation?

Cheers!