Validating XML against XSD seems to fail

Hi there!

First, thank you very much for this awesome library.

I tried to validate the XML output against the XSD [1] and found there was the following error:
Error: The ‘http://www.omg.org/spec/BPMN/20100524/MODEL:definitions’ element is not declared. Line 2, Position 2.

Thanks for your help!

Best regards,
-Bruno.

[1] https://www.omg.org/spec/BPMN/20100501/Semantic.xsd

Validate against bpmn20.xsd (including all the related schemas)