How to validate Peppol BIS 3 invoices in 5 lines of Python (or Node, or Go)

Starting September 2026 every B2B invoice in France must be e-invoiced (Peppol/Factur-X). Germany has mandated XRechnung for B2G since 2020. Italy has been on FatturaPA since 2019. Spain's Verifactu rolls out 2025-2026. If you build accounting software, an ERP, or any e-commerce flow that touches EU customers, you'll likely need to validate these invoice formats at some point. The two main open-source Schematron engines are phive (by Philip Helger of the EN16931 working group) and Mustang (by Jo