Is "What things are safe?", instead of "which things are safe?", grammatical and good usage? Here is an example in a (simplified) context:

In this formula, we use predicate Psafe, which describes what values are safe.

In this formula, we use predicate Psafe, which describes which values are safe.

I prefer the latter, while a coauthor prefers the former to avoid repeating the word "which".

FWIW: In the actual text, we already given technical definitions for the collection of "values" and the concept of "safe". And "predicate" is used in the sense from logic — as a property that can be true or false of some subject (here, if v is a value then Psafe(v) can be true or false). So Psafe are selecting a subcollection of "safe" values across a larger collection of all values.

  • (FWIW, I've already found english.stackexchange.com/a/3414/71325 and english.stackexchange.com/a/59350/71325, but I don't find it very convincing in this case). – Blaisorblade Mar 8 '20 at 15:37
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    I'm with you here, and if it weren't for the repeat, 'which' would be my firm favourite. But in speaking, more weight / stress is given to the first 'which' (- - ....), so the repeat is a trivial issue. And actually, 'what' sounds a little clumsy here. – Edwin Ashworth Mar 8 '20 at 16:13
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    Your coauthor has some erroneous ideas about natural use of English, if he thinks you should avoid repeating which in such contexts. Native speakers routinely do this without anyone noticing anything unusual (even more so with repeated that in similar constructions). – FumbleFingers Mar 8 '20 at 18:01
  • ... Even FumbleFingers can begin to sound almost normal. – Edwin Ashworth Mar 8 '20 at 19:14
  • "In this formula, we use predicate Psafe, which describes safe values." – Greybeard Mar 8 '20 at 20:05

I would write:

In this formula, we use the predicate 'Psafe' which describes the values that are safe.

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