public void ResolveExtensions()
        {
            var extDefn = source.GetExtensionDefinition("http://hl7.org/fhir/StructureDefinition/data-absent-reason");
            Assert.IsNotNull(extDefn);
            Assert.IsInstanceOfType(extDefn, typeof(StructureDefinition));

            try
            {
                extDefn = source.GetExtensionDefinition("http://hl7.org/fhir/StructureDefinition/Patient");
                Assert.Fail();
            }
            catch
            {
                ;
            }
        }