ns2:Element1[ns2:Element2 = ns1:Element2]

selects those ns2:Element1 nodes that have an [ns2:Element2 child that
is equal to a ns1:Element2 child.

but your sample input has no ns2:Element1 elements that have
ns1:Element2 children.

By evaluating it in a variable you are evaluating it in a different
context.

David

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