baseset: implement a fastasc and fastdesc Baseset contains already-computed revisions. It is considered "cheap" to do operations on an already-computed set. So we add attributes to hold version of the list in ascending and descending order and use them for `fastasc` and `fastdesc`. Having distinct lists is important to provide correct iteration in all cases. Altering a python list will impact an iterator connected to it. eg: not preserving order at iterator creation time >>> l = [0, 1] >>> i = iter(l) >>> l.reverse() >>> list(i) [1, 0] eg: corrupting in progress iteration >>> l = [0, 1] >>> i = iter(l) >>> i.next() 0 >>> l.reverse() >>> i.next() 0