How to get n-hops bidirectional in sparql

Question

I am using the following query to get subclass of predicate up and down for a given concept in wikidata.

SELECT ?item ?itemLabel ?linkTo {
  { wd:Q22673982 wdt:P279* ?item } UNION { ?item wdt:P279* wd:Q22673982 }
  OPTIONAL { ?item wdt:P279 ?linkTo }
  SERVICE wikibase:label {bd:serviceParam wikibase:language "en" }
}

In order to limit the hops only to 10, I extended the aforementioned query into the following.

PREFIX gas: <http://www.bigdata.com/rdf/gas#>

SELECT ?item ?itemLabel ?linkTo {
  { SERVICE gas:service {
    gas:program gas:gasClass "com.bigdata.rdf.graph.analytics.SSSP" ;
                gas:in wd:Q22673982 ;
                gas:traversalDirection "Forward" ;
                gas:out ?item ;
                gas:out1 ?depth ;
                gas:maxVisited 10 ;
                gas:linkType wdt:P279 .
  } } UNION { SERVICE gas:service {
    gas:program gas:gasClass "com.bigdata.rdf.graph.analytics.SSSP" ;
                gas:in wd:Q22673982 ;
                gas:traversalDirection "Reverse" ;
                gas:out ?item ;
                gas:out1 ?depth ;
                gas:maxVisited 10 ;
                gas:linkType wdt:P279 .
  } }
  OPTIONAL { ?item wdt:P279 ?linkTo }
  SERVICE wikibase:label {bd:serviceParam wikibase:language "en" }
}

I tried to do it in a similar way using DBpedia as follows.

SELECT * {
  { dbr:Word2vec dct:subject* ?item } UNION { ?item dct:subject* dbr:Word2vec }
  OPTIONAL { ?item dct:subject ?linkTo }
}

However, I get the following error: Virtuoso 37000 Error SP031: SPARQL compiler: Variable '_::trans_subj_4_0' is used in subexpressions of the query but not assigned

My question is; Is it possible to do the same in DBpedia?

I am happy to provide more details if needed.

Answer 1

The Virtuoso 37000 Error SP031: SPARQL compiler:... error is flagged in a couple of open issues (#530, #681) on the github-hosted Virtuoso project.

As noted there, your query will likely run if you simply un-tick the Strict checking of void variables box on the SPARQL query form.