Python: How do I access NamedRerouteDeclaration from a given NamedRerouteUsage?

Hi there,
is there a way in Python to access the NamedRerouteDeclaration Expression from a given NamedRerouteUsage Expression? I cannot find anything in the documentation about it…

My Samplecode so far: recursively check material expressions
Sample Material: Sample material with reroute expressions

Thanks in advance!