Sorted sets in PADS

I've updated my library of Python algorithm implementations to include another module: SortedSet. It maintains a set of items (like a Python set data type), but adds a method to report all the items in the set in sorted order.

The obvious ways of doing this are to sort everything at reporting time or to maintain the items in a balanced binary tree data structure. My solution combines the simplicity of the first solution with the asymptotic speed of the second: store, along with the set itself, the sorted output from the last report and smaller lists of changed items. A new report request can be handled by sorting the smaller lists and merging them with the previous output, in constant time for each previously listed item and logarithmic time for each changed item.

Comments:

self._removals = set()
self._additions = set()
This may be more friendly for the GC:
self._removals.clear()
self._additions.clear()

def __init__(self,comparison=None):
Better to put a space after the commas (as PEP8 too says):
def __init__(self, comparison=None):

def __init__(self, comparison=None):
    """Create a new sorted set with the given comparison function."""
    self._comparison = comparison
    self._set = set()
    self._previous = None
    self._removals = set()
    self._additions = set()