public IEnumerable <SyntaxNode> GetNodes(SyntaxGenerator gen)
{
var contents = Usings.OfType <IHasSyntaxNodes>().Concat(
Members.OfType <IHasSyntaxNodes>()).Select(m => m.GetNodes(gen))
.SelectMany(n => n);
yield return(gen.NamespaceDeclaration(Name, contents));
}