我的计算器我想要实现连加功能，请问怎么修改啊，帮帮忙



Option Explicit
Dim startyunsuan As Boolean '是否开始运算
Dim yunsuan As Long '运算标志
Dim firstyunsuan As Double '先前的操作数

Private Sub cmd09_Click(Index As Integer)
If Val(txtDisp.Text) <= 0 Then '数字0-9
txtDisp.Text = Index
Else
txtDisp.Text = txtDisp.Text & Index
End If
End Sub

Private Sub cmdBackspace_Click()
On Error Resume Next
txtDisp = Left(txtDisp, Len(txtDisp) - 1) '退格键
End Sub

Private Sub cmdFuhao_Click(Index As Integer)
Select Case Index

Case 0

yunsuan = 1

Evaluate

Case 1

yunsuan = 2

Evaluate

Case 2

yunsuan = 3

Evaluate

Case 3

yunsuan = 4

Evaluate

Case 4

DisposeResult

End Select

End Sub

Private Sub Evaluate() '四则运算赋值

firstyunsuan = Val(txtDisp)

startyunsuan = True
txtDisp = ""

End Sub

Private Sub DisposeResult() '处理计算结果

On Error GoTo ToExit '打开错误陷阱

Select Case yunsuan '操作标志

Case 1

txtDisp = firstyunsuan + Val(txtDisp)

Case 2

txtDisp = firstyunsuan - Val(txtDisp)

Case 3

txtDisp = firstyunsuan * Val(txtDisp)

Case 4

txtDisp = firstyunsuan / Val(txtDisp)

End Select

yunsuan = 0 '操作标志清0

Exit Sub

ToExit:

MsgBox "除数不能为0！", vbOKOnly, "Error" '提示出错

On Error Resume Next '遇到错误不管，继续执行

End Sub

Private Sub cmdClearAll_Click() 'C按钮清除所有的运算结果

yunsuan = 0
txtDisp = "0"

End Sub

Private Sub cmdClearLastInput_Click() 'CE按钮清除最后一次输入的数字

txtDisp = "0"

End Sub

Private Sub txtDisp_KeyPress(KeyAscii As Integer)
If KeyAscii < 47 Or KeyAscii > 57 Then '文本框只允许输入数字0-9还有小数点
KeyAscii = 0
End If
End Sub