public void FillTest()
        {
            Document doc = new Document();
            doc.Fill(
                Width => 100,
                Height => 200,
                Attributes => new Pairs{ attr1 => 1,
                                         attr2 => 2 });

            Assert.AreEqual(100, (int)doc["Width"]);
            Assert.AreEqual(200, (int)doc["Height"]);
            Assert.IsInstanceOfType(doc["Attributes"], typeof(Document));
        }