tibbe/unordered-containers — pull request #50 by michalt (2012-10-18 17:44:51+00:00)
Title: Collapse nodes with one Leaf/Collision child (issue #47)

Collapses `BitmapIndexed` and `Full` with just one `Leaf` or `Collision` child.

Note that in `delete` the line:
```
Empty | A.length ary == 1 -> Empty
```
is unnecessary if we decide that the collapsing should be an invariant. That's because it would correspond to a situation where the considered `BitmapIndexed` had just one key-value pair at the end (and thus should have been represented as a `Leaf` in the first place).