Require that happens_after is not mutable (#866)
* Require that happens_after is not mutable
* Tweak type tests for happens_after
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Co-authored-by: Andreas Kloeckner <inform@tiker.net>
* Require that happens_after is not mutable
* Tweak type tests for happens_after
---------
Co-authored-by: Andreas Kloeckner <inform@tiker.net>