public void TestToString1()
        {
            var prefixes = new[] {
                SPARQL.Prefix("p1", "http://test.com/p1"),
                SPARQL.Prefix("p2", "http://test.com/p2"),
            };



            var source = TestDataProvider.GetQuerable <dynamic>("", prefixes: prefixes);

            var query = source.Prefix("p3", "http://test.com/p3")
                        .Prefix("p4", "http://test.com/p4")
                        .Prefix("p5", "http://test.com/p5");

            var resStr = query.ToString();

            resStr.Should().StartWith(
                @"PREFIX p1: <http://test.com/p1>
PREFIX p2: <http://test.com/p2>
PREFIX p3: <http://test.com/p3>
PREFIX p4: <http://test.com/p4>
PREFIX p5: <http://test.com/p5>"
                );
        }
        /// <summary>
        /// Evaluates Prefix expression
        /// </summary>
        /// <param name="m">method call expression</param>
        private void VisitPrefix(MethodCallExpression m)
        {
            string prefix = (string)((ConstantExpression)m.Arguments[1]).Value;
            string iri    = (string)((ConstantExpression)m.Arguments[2]).Value;

            Prefixes.Add(SPARQL.Prefix(prefix, iri));
        }