public void AddUsing(UsingClause un)
        {
            if (DeclarationFound){
                Compiler.Report.Error (1529, un.Location, "A using clause must precede all other namespace elements except extern alias declarations");
            }

            if (clauses == null)
                clauses = new List<UsingClause> ();

            clauses.Add (un);
        }
 void Warning_DuplicateEntry(UsingClause entry)
 {
     Compiler.Report.Warning (105, 3, entry.Location,
         "The using directive for `{0}' appeared previously in this namespace",
         entry.ResolvedExpression.GetSignatureForError ());
 }