As can be seen in
https://travis-ci.org/HOL-Theorem-Prover/HOL/jobs/124717527
our use of Posix waiting is occasionally causing a Fail exception to be raised from inside the Posix implementation (there's no Fail exception with that message in our source code, and our code *is* doing a wait at that point).
I'm afraid this doesn't seem easy to reproduce.
Michael
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