按键精灵9 清除给变量赋值的字符串

Hwnd = Plugin.Window.GetKeyFocusWnd()
sRect = Plugin.Window.GetClientRect(Hwnd)
WinArray=Split(sRect,"|")
GetColor=GetPixelColor(x,y)
For 3
y=WinArray(1)+421
For 12
x = WinArray(0) + 713
For 7
If GetColor = "FFFFFF" Then
i="0"
Else
i = "1"
End If
a = a & i
x=x+1
Next
y=y+1
Next
y=y+33
Call Plugin.File.WriteFileEx("C:\Documents and Settings\Administrator\桌面\新建 文本文档.txt", a)
Next
请问大侠们如何清除给a赋值的字符串

a =""
这样就行