def _select_uris_and_graphs(sparql, opts={})
      select_results = Tripod::SparqlClient::Query.select(sparql)
  
      uri_variable = opts[:uri_variable] || 'uri'
      graph_variable = opts[:graph_variable] || 'graph'

      return [] unless select_results.select{|r| r.keys.length > 0 }.any?
    
      select_results.reduce([]) do |memo, result|
        u = result[uri_variable]['value']
        g = result[graph_variable]['value'] if result[graph_variable]
        memo << [u, g]
        memo
      end
    end
def _select_uris_and_graphs(sparql, opts={})
      select_results = Tripod::SparqlClient::Query.select(sparql)
  
      uri_variable = opts[:uri_variable] || 'uri'
      graph_variable = opts[:graph_variable] || 'graph'

      return [] unless select_results.select{|r| r.keys.length > 0 }.any?
    
      select_results.reduce([]) do |memo, result|
        u = result[uri_variable]['value']
        g = result[graph_variable]['value'] if result[graph_variable]
        memo << [u, g]
        memo
      end
    end
based on the query passed in, build an array of [uri, graph] pairs
 @param [ String] sparql. The sparql query
 @param [ Hash ] opts. A hash of options.

 @option options [ String ] uri_variable The name of the uri variable in the query, if not 'uri'
 @option options [ String ] graph_variable The name of the uri variable in thh query, if not 'graph'