How to access the elements maintaining external references

[Alive59]

Hello, thanks for the nice library. But I met some problems while reading my GML file.

For example, I have a GML file with such a member:
core:cityObjectMember
<frn:CityFurniture gml:id="frn_0fede2e1-e9b0-4c85-bf79-f0f0561088da">
<frn:class codeSpace="../../codelists/CityFurniture_class.xml">1000</frn:class>
<frn:function codeSpace="../../codelists/CityFurniture_function.xml">1120</frn:function>
frn:lod3Geometry
gml:MultiSurface
gml:surfaceMember
<gml:Polygon gml:id="fme-gen-c0bb7ba3-21dc-47c1-afac-c9463a5fb91a">
gml:exterior
<gml:LinearRing gml:id="fme-gen-c0bb7ba3-21dc-47c1-afac-c9463a5fb91a_0">
gml:posList35.08438112805094 138.85740168268748 1.943731604668821 35.08433208088209 138.8574225819179 1.9891862646404748 35.084333421831275 138.8574272394311 1.989130431634036 35.084382469044485 138.85740634018458 1.9436757312691486 35.08438112805094 138.85740168268748 1.943731604668821</gml:posList>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</gml:surfaceMember>
</gml:MultiSurface>
</frn:lod3Geometry>
</frn:CityFurniture>
</core:cityObjectMember>

I successfully accessed the geometry child (frn:lod3Geometry), but failed to access those with external linkage (frn:class, frn:function). Are there any existing APIs for this purpose?

Thank you.