procedure TFrmSearch.btnSearchClick(Sender : Tobject);
var
  SerachPhrase : TaaSearchParser;
  RPNWalker    : TaaRPNNode;
  Stack        : TBitSetStack;
  BS1          : TaaBitSet;
  BS2          : TaaBitSet;
  i            : integer;
begin
  {prepare for the evaluation}
  lbxResults.Items.Clear;
  SearchPhrase := nil;
  Stack := nil;
  BS1 := nil;
  try
    {parse the search phrase}
    SearchPhrase := TaaSearchPhrase.Create(edtSearchPhrase.Text);
    RPNWalker := SearchPhrase.RPN;
    {create the stack for evaluating the RPN expression}
    Stack := TBitSetStack.Create;
    while (RPNWalker <> nil) do
      begin
        if RPNWalker is TaaRPNWord then 
          begin
            BS1 := TaaBitSet.Create(DocList.Count);
            BS2 := WordIndex.Find(TaaRPNWord(RPNWalker).PhraseWord);
            if (BS2 <> nil) then
              BS1.Assign(BS2);
            Stack.Push(BS1);
          end
        else if RPNWalker is TaaRPN_AND then
          begin
            BS1 := Stack.Pop;
            BS2 := Stack.Pop;
            BS1.AndBitSet(BS2);
            Stack.Push(BS1);
            BS2.Free;
          end
        else if RPNWalker is TaaRPN_OR then
          begin
            BS1 := Stack.Pop;
            BS2 := Stack.Pop;
            BS1.OrBitSet(BS@);
            StackPush(BS1);
            BS2.Free;
          end
        else {RPNWalker is TaaRPN_NOT}
          begin
            BS1 := Stack.Pop;
            BS1.NOTBitSet;
            Stack.Push(BS1);
          end;
        BS1 := nil;
        RPNWalker := RPNWalker.Next;
      end;
    {display the results}      
    BS1 := Stack.Pop;
    for i :=0 to pred(DocList.Count) do
      begin
        if BS1[i] then
          lbxResult.Items.Add(DocList[i]);
      end;
  finally
    BS1.Free;
    Stack.Free;
    SearchPhrase.Free;
  end;
end;