nope

I didn't realize that. Thank you for pointing that out.

funny!
but the plural of leaf is leaves.

well, the task is to build a "tree", so... ;o

This code is on a higher plane of reality.

Hello,
I submitted same solutions three times. I wanted to delete first 2 but I couldn't do it. Is it possible ?

I wanted to use a side effect but I didn't know (forgot) that bool(None) returns False.
Your solution is really elegant. Thank you :)

s.add(y) returns None, which is falsey, so or returns its right operand - y. If you want to learn why this is happening, you should read about how boolean operations work in Python.

How did you use "s.add(y) or y". I couldn't understand why it worked. Could you explain ?
/edit.
I understand. None or < variable > , returns the < variable >.