两个图元集合进行减操作

<P>有遇到问题了，请高手指点如何把两个ISelection图元集合进行减操作？</P>
<P>    Dim i As Long
    Dim pMap As IMap
    Dim pActiveView As IActiveView
    Dim pFeatureLayer As IFeatureLayer
    Dim pGeoFeatureLayer As IGeoFeatureLayer
    Dim pFeatureSelection As ISelection
    Dim pAllFeatureSelection As ISelection</P>
<P>'首先记录下指定图层当前的FeatureSelection集合
    Set pFeatureSelection = pMap.FeatureSelection
    '清除选择
    pMap.ClearSelection
    
    '选择指定图层的所有图元
    Set pActiveView = pMap
    pMap.SelectByShape pActiveView.FullExtent, Nothing, False
    
    '记录下所有图元的集合
    Set pAllFeatureSelection = pMap.FeatureSelection
    '清除选择
    pMap.ClearSelection
    '在所有图元中去掉当前所选择的图元，得到取反后的图元并添加到地图选择集合当中
    'Set pMap.FeatureSelection = pAllFeatureSelection - pFeatureSelection</P>
<P>    '上面的语句是伪语言，如何实现它所秒素的功能？

    pMap.FeatureSelection.SelectAll
    pActiveView.PartialRefresh esriViewGeoSelection, Nothing, Nothing
    </P>