退格键的那个代码怎么编啊

Dim flag As Boolean
Dim addFlag As Boolean

Private Sub Command1_Click()              '加法
    Static a As Long, b As Long
    If Text1 = "" Then
        MsgBox "error"
        Exit Sub
    End If
    If addFlag Then
        b = Val(Text1)
        Text1 = a + b
        a = Val(Text1)
    Else
        addFlag = True
        a = Val(Text1)
    End If
    Text1.SetFocus
    flag = False
End Sub


Private Sub Text1_KeyPress(KeyAscii As Integer)   ‘过滤非数字字符
    If KeyAscii < 49 Or KeyAscii > 57 Then
        KeyAscii = 0
    End If
    If Not flag Then
        flag = True
        Text1 = ""
    End If
End Sub

等于的按钮过程和加法相似，自己多琢磨下就出来了