Not Found UNDO in my RichEdit with Syntax Highlighting

I have created a descending class of TRichEdit for the syntax highlighting. the code of the main procedure is Procedure SyntaxHihglighting; the procedure analyzes a line and it transforms it into format RTF.  

class TRichEditPHP = (TRichEdit)
private
   ...
   OutStream : TMemoryStream;
   Procedure SyntaxHighlighting;
protected
    procedure Change; override;
public
   ...
published
   ...
end;
   
Procedure TRichEditPHP.SyntaxHighlighting;  
var  
      ...  
begin  
      OutStream := TMemoryStream.create;  
      ...  
      ...  
      / / end analysis lines, we take it to the exit  
      OutStream.Position := 0;  
      lines.strings[fila-1] := ConvertStreamString(OutStream);  
      OutStream.Free;  
      ...  
End;  
 
This procedure places it in the event Change.  
 
procedure TRichEditPHP.Change;  
begin  
  if HandleAllocated then  
  begin  
    if not ActivaConversion then  
    begin  
      UpdateCursorPos;  
      SyntaxHighlighting;  
    end;  
  end;  
  inherited;  
end;  
 
My Problem is when I use my class in a program the procedure UNDO it doesn't work it is disabled while I go writing text in the TRichEditPHP1.  
As I can solve it, another alternative exists to UNDO creating another procedure.
Please, downloading my Demo Program whith source code: http://www.malpicasoft.com/english/demo/DemoEnglish.zip