In Shacl4bib there should be a condition: if field existing then check subfield

[pkiraly]

Sometimes a validity constraint looks like: "subfield $a should always be available in field X". It could be translated as

• if X is not available, then $a is also not available, but it is not an issue, the test should return NA instead if FAILED
• if X is available, then $a must be available, so the test should FAIL

Now it is not possible to make this distinction.