Until now einvoice-preflight could say a Dutch invoice had the Peppol NL-R problems, but not the NLCIUS ones, the Dutch national format (SI-UBL 2.0) that government buyers and many accounting packages ask for. The rules were published only as Schematron compiled to XSLT, inside a Maven Central artefact.
The new build-nl-rules.mjs reads that XSLT with the same reader used for the Romanian and Singapore rules, keeps only the pattern holding the Dutch checks (39 of 1,022; it refuses to run if base rules are mixed in) and ships them as an add-on that runs after the EU base. They fire only for a Dutch seller whose customization ID names NLCIUS.
On a made-up invoice with no seller street, no buyer or order reference and a payment code of 10, the check reports BR-NL-3, BR-NL-2 and BR-NL-12, each with a plain-English fix, plus the warning that a country subdivision is ignored on receipt. The clean example passes with no rule skipped.
Not done: the CII form, the Saxon route, and any real Dutch file or Peppol access point verdict. The examples are ours.