[glunn]

Reference: #546

[CodeByZach]

Thanks for this PR. Your key filter in extend is the basis of the fix in 1.3.0, together with two other changes, and you're credited in GHSA-5fmc-v4p3-pqpg. We didn't merge this branch itself because it also renames __extends to __s in one place, which makes pace throw when it loads. Closing in favor of the release.