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* Add example in docs
* Don't store previous sources value (there is none)

Change-Id: I5b876905d4117a7f9d1966a70ce7134aef3110b8
Reviewed-on: https://chromium-review.googlesource.com/c/1455340
Commit-Queue: David Turner <digit@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: David Turner <digit@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629580}