Github user ajs6f commented on the issue:
https://github.com/apache/incubator-commonsrdf/pull/23
The comment on Jena's `Node_Variable`:
```
"variable" nodes; these are outside the RDF2003 specification, but are
used internally for "placeholder" nodes where blank nodes would be
wrong, most specifically in Query.
```
leads me to believe that we are talking about SPARQL here. There's not
really any other place in Jena I know about that would use such machinery-- the
`find` methods on various structures just use `null` or `Node_ANY` as a
wildcard.
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