It seems that the purpose of this method is to wait until enough nodes become available to retrieve with a given treasure map. But in its current state it has a few problems:

• The name does not really convey what it does. Something like prepare_for_retrieval() would be more straightforward.
• The docstring is misleading. It says "Determines which Ursulas are known and which are unknown." We don't really use its return value, its main purpose is to wait until Ursulas are known. The user should be given a clear strategy of how this method is supposed to be used.
• It takes a decrypted treasure map, which is not trivial to obtain (with public API)
• Might be more intuitive if by default it waited for threshold Ursulas to be online, not all of them - that's what we really need