Ah. My mistake, then. I don't happen to have the card with me at the moment, but my recollection was that the original rules prohibited building a tree onto an existing nest but not onto an existing singleton. Were that the case, you would need to be able to distinguish a nest from a singleton.
Ah, this is in fact the case, you're right. However, given that this information is not meant to be secret, it's easy to just lift the piece before playing to check if there's a piece underneath if a reminder is necessary. But that's the only time one might need to look under - just to ensure one isn't trying to form a tree on an existing nest. Timothy