def graph(graph_uri, &block)

      if block_given?
        self.graph_lambdas ||= []
        self.graph_lambdas << block
        self
      else
        self.graph_uri = graph_uri.to_s
        self
      end
    end
def graph(graph_uri, &block)

      if block_given?
        self.graph_lambdas ||= []
        self.graph_lambdas << block
        self
      else
        self.graph_uri = graph_uri.to_s
        self
      end
    end
Restrict this query to the graph uri passed in
 You may also pass a block to an unbound graph, ?g
 then chain a where clause to the criteria returned to bind ?g

 @example .graph(RDF::URI.new('http://graphoid')
 @example .graph('http://graphoid')
 @example .graph(nil) { "?s ?p ?o" }.where("?uri ?p ?g")

 @param [ String, RDF::URI ] The graph uri
 @param [ Block ] A string to be executed within an unbound graph, ?g

 @return [ Tripod::Criteria ] A criteria object