I'm trying to run this query on my instance of Virtuoso against the DBpedia Virtuoso SPARQL endpoint
```
SELECT ?s1 ?sc
WHERE {
  SERVICE <http://dbpedia.org/sparql> {
    SELECT ?s1 ((?sc * 3e-1) as ?sc) ?o1
    WHERE {
      ?s1 ?s1textp ?o1 .
      FILTER (CONTAINS(?o1, 'MARTIN AND LUTHER AND PRAISED'))
    }
    ORDER BY DESC(?sc + sql:rnk_scale(<LONG::IRI_RANK>(?s1)))
    LIMIT 50 OFFSET 0
  }
}
```

After running I receive this error message: 
> `Virtuoso RDFZZ Error DB.DBA.SPARQL_REXEC('http://dbpedia.org/sparql', ...) returned Content-Type 'text/plain' status 'HTTP/1.1 400 Bad Request`
>
> `Virtuoso 37000 Error SP031: SPARQL compiler: Free-text options can be specified only for triple patterns with special predicates, not for plain patterns`

I'm unsure about special predicates and plain patterns, what I'm trying to achieve is to do text search similar to searching on the DBpedia endpoint but instead using their endpoint as a service.
I would appreciate any guidance. Thank you!
Sparql Endpoint text query search
This is a GitHub Issue
repo:virtuoso-opensource
owner:openlink
Title : Sparql Endpoint text query search
Issue date:

--- start body ---
I'm trying to run this query on my instance of Virtuoso against the DBpedia Virtuoso SPARQL endpoint
```
SELECT ?s1 ?sc
WHERE {
  SERVICE <http://dbpedia.org/sparql> {
    SELECT ?s1 ((?sc * 3e-1) as ?sc) ?o1
    WHERE {
      ?s1 ?s1textp ?o1 .
      FILTER (CONTAINS(?o1, 'MARTIN AND LUTHER AND PRAISED'))
    }
    ORDER BY DESC(?sc + sql:rnk_scale(<LONG::IRI_RANK>(?s1)))
    LIMIT 50 OFFSET 0
  }
}
```

After running I receive this error message: 
> `Virtuoso RDFZZ Error DB.DBA.SPARQL_REXEC('http://dbpedia.org/sparql', ...) returned Content-Type 'text/plain' status 'HTTP/1.1 400 Bad Request`
>
> `Virtuoso 37000 Error SP031: SPARQL compiler: Free-text options can be specified only for triple patterns with special predicates, not for plain patterns`

I'm unsure about special predicates and plain patterns, what I'm trying to achieve is to do text search similar to searching on the DBpedia endpoint but instead using their endpoint as a service.
I would appreciate any guidance. Thank you!
--- end body ---
