union causes invalid mixed content

[VladimirAlexiev]

Take https://www.openaire.eu/schema/1.0/oaf-common-1.0.xsd. It has this:

	<xs:simpleType name="boolOrEmptyType">
		<xs:union memberTypes="emptyType xs:boolean"/>
	</xs:simpleType>

According to the RelaxNG book sec 16.2.2 Compatibility with W3C XML Schema, this should be translated to

element foo{emptyType|xs:boolean}

XSDtoRNC.xsl almost does it:

	<rng:define name="boolOrEmptyType">
		<rng:choice><rng:ref name="emptyType"/>
                  emptyType xs:boolean
               <rng:data type="boolean"/></rng:choice>
	</rng:define>

but fails because the emptyType xs:boolean part is invalid mixed content.

My XSL skills are rudimentary. I looked at https://github.com/epiasini/XSDtoRNG/blob/master/xsdtorngconverter/XSDtoRNG.xsl#L429 but can't find the bug...