tibbe/unordered-containers — pull request #46 by michalt (2012-10-09 17:54:30+00:00)
Title: Don't leave a BitmapIndexed with only one child (issue #39).

In some cases `filter` and `delete` would leave a BitmapIndexed with only one child. But when the map was rebuild using `fromList . toList` then the internal representation of the tree was obviously slightly different. This broke equality since it assumes that equal maps are also structurally equal.