What or which before plural noun

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.

I would write:

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