[sv-ac] When does an assertion fail vacuously?

Where is vacuous fail defined in the LRM? I could not find it.

Consider the property (*not* (a |-> b)), and "a"==0; Then (a|-> b) is
vacuous success.

Does that mean that *not*(a|->b) is vacuous fail?

Also, at our meeting today, I heard that* accept_on* can produce vacuous
fail. I don't get it.

Can you point me to cases of vacuous failure, and where in the LRM this
defintion exists?

16.15.8 defines nonvacuous, but the LRM does not define vacuous (I don’t
think).
I can't find vacuous fail or vacuous pass. It can succeed vacuously, but
that is not a pass or a fail.
Also, here is a pass counter, fail counter, vacuous counter.

Thanks,
Ben

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