Questions about mayaUsdLib.PrimWriter

[frankchen211]

Q1:
I got a custom PrimWriter for assemblyReference node, before registered my Writer I exported some geoms, then I got this error when registering:

...maya-usd/lib/mayaUsd/fileio/primWriterRegistry.cpp : 'Multiple writers for type assemblyReference'

How to correctly register my custom one?

try:
    mayaUsdLib.PrimWriter.Register(primAssemblyReferenceWriter, "assemblyReference")
    # do export
except Exception as e:
    logger.exception(e)
finally:
    mayaUsdLib.PrimWriter.Unregister(primAssemblyReferenceWriter, "assemblyReference")

Q2:
How to skip exporting descendent nodes when the assembly reference representation is a scene(transforms and meshes)?
I would like to process those export later into separate usd files.

I customized a writer for mesh, but I can't find the method inside the writer to determine which mesh to export by conditions.
e.g.: only meshes/transforms in the assembly reference nodes will be excluded from exporting, otherwise inherit the default export methods.

Also I noticed that although I haved unregistered the customized writer then use the default exporter from Maya file menu, no mesh is exported.