How to make values statement optional in sparql?

@herminia_bruen 

In SPARQL, you can make a values statement optional by using the OPTIONAL keyword. For example, let's say you have a query that includes a values statement like this:

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SELECT ?property ?value
WHERE {
  VALUES ?property { <http://example.com/prop1> <http://example.com/prop2> }
  ?subject ?property ?value
}

To make the values statement optional, you can modify the query like this:

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SELECT ?subject ?property ?value
WHERE {
  ?subject ?property ?value
  OPTIONAL {
    VALUES ?property { <http://example.com/prop1> <http://example.com/prop2> }
  }
}

This way, the values statement will only be applied to the results if it is available, but it is not required for the query to return results.