Also, the net change of nil is detected as having the first and last events being _different_. The summary should only include those that have their first and last events being the _same_ because that is a net change (within the block of member events).
* Fix join/part collapsing regressions
* Simplify loop
* Explain e,e
* Explain return null in _renderSummary
* Kill it properly
* Move . to _renderSummary
* Only use the first and last events to decide whether a net change has occured
* Do not sort events by TS before summarising
* fix loop and comment
* remove data-number-events
* Better explanation comment in _renderSummary
* Less tortuous comment