private void ProcessPostfix(Control ctl)
        {
            if (ExprHelper.IsOperator(ctl.Text))
            {

                if (_preCtl != null && ExprHelper.IsOperator(_preCtl.Text))
                {
                    if (ctl.Text == "-")
                    {
                        _nextCtl.Text = ctl.Text   _nextCtl.Text;
                        Output(_nextCtl);
                    }
                    else if (ExprHelper.IsUnaryFunction(ctl.Text))
                    {
                        PushStack(ctl);
                    }
                }
                else
                {
                    while (pnlStack.Controls.Count > 0 &&
                        ExprHelper.GetPriority(ctl.Text) <= ExprHelper.GetPriority(PeekStack().Text))
                        Output(PopStack());
                    PushStack(ctl);
                }
            }

            else if (ctl.Text == "(")
                PushStack(ctl);
            else if (ctl.Text == ")")
            {
                Control x = PopStack();
                while (x.Text != "(")
                {
                    Output(x);
                    x = PopStack();
                }
            }
            else // IsOperand
            {
                Output(ctl);
            }

        }

        Control PeekStack()
        {
            return pnlStack.Controls[pnlStack.Controls.Count - 1];
        }
        Control PopStack()
        {
            Control ctl = pnlStack.Controls[pnlStack.Controls.Count - 1];
            pnlStack.Controls.Remove(ctl);

            return ctl;
        }
        void PushStack(Control ctl)
        {
            pnlStack.Controls.Add(ctl);

            if (pnlStack.Controls.Count > 1)
                pnlStack.Controls[pnlStack.Controls.Count - 2].BackgroundImage = _imgNormal;
        }
        void Output(Control ctl)
        {
            pnlOutput.Controls.Add(ctl);

            if (pnlOutput.Controls.Count > 1)
                pnlOutput.Controls[pnlOutput.Controls.Count - 2].BackgroundImage = _imgNormal;
        }