I have this federated query with `SERVICE`. I want to use `VALUES` to test a number of combinations of sources.

This query below works
```sparql
SELECT * WHERE { 
   
    SERVICE <http://localhost:8890/sparql?default-graph-uri=http://www.source1.fr/> {
            ?s1 foaf:name ?o1.
        } .

        SERVICE <http://localhost:8890/sparql?default-graph-uri=http://www.source2.fr/> {
            ?s1 foaf:name ?o1.
        } .
}
```
but this one doesn't:

```sparql
SELECT * WHERE { 
    {
        SELECT ?bgp1 ?bgp2 WHERE {
            VALUES ( ?bgp1 ?bgp2 ) { ( <http://localhost:8890/sparql?default-graph-uri=http://www.source1.fr/> <http://localhost:8890/sparql?default-graph-uri=http://www.source2.fr/>  ) } 
           
        } 
    }
    {
    SERVICE ?bgp1 {
            ?s1 foaf:name ?o1.
        } .

        SERVICE ?bgp2 {
            ?s1 foaf:name ?o1.
        } .
    }
}
```

It just gives me a mysterious error `Unable to use variable ?bgp1 in OPTION () clause, try to rephrase the query`.
Cannot use `SERVICE` with variable
This is a GitHub Issue
repo:virtuoso-opensource
owner:openlink
Title : Cannot use `SERVICE` with variable
Issue date:

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I have this federated query with `SERVICE`. I want to use `VALUES` to test a number of combinations of sources.

This query below works
```sparql
SELECT * WHERE { 
   
    SERVICE <http://localhost:8890/sparql?default-graph-uri=http://www.source1.fr/> {
            ?s1 foaf:name ?o1.
        } .

        SERVICE <http://localhost:8890/sparql?default-graph-uri=http://www.source2.fr/> {
            ?s1 foaf:name ?o1.
        } .
}
```
but this one doesn't:

```sparql
SELECT * WHERE { 
    {
        SELECT ?bgp1 ?bgp2 WHERE {
            VALUES ( ?bgp1 ?bgp2 ) { ( <http://localhost:8890/sparql?default-graph-uri=http://www.source1.fr/> <http://localhost:8890/sparql?default-graph-uri=http://www.source2.fr/>  ) } 
           
        } 
    }
    {
    SERVICE ?bgp1 {
            ?s1 foaf:name ?o1.
        } .

        SERVICE ?bgp2 {
            ?s1 foaf:name ?o1.
        } .
    }
}
```

It just gives me a mysterious error `Unable to use variable ?bgp1 in OPTION () clause, try to rephrase the query`.
--- end body ---
