Search by "xref" - which values are allowed?

[elenaheister-cap]

I would like to retrieve compounds by means of a cross reference, for example BiGG, ChEBI, KEGG compound or seed compound.

PubChem clearly offers this functionality as explained in their docs, but only provides one example using "PatentID" as the xref type.

Equally, this repo offers the functionality but again just points to the PUG REST API documentation here and uses the same example than above in its unit test.

If anyone could provide information on which values are possible, it would be hugely appreciated. These could be added to PubChemPy's docs and/or the code itself.