Grouping changes through VS-Extension in undo stack

Hello,

I´ve wrote a extension based on this solution.

i get the buffer for the whole document by

TextView.GetBuffer(out IVsTextLines pBuffer);

and delete lines with EnvDTE.EditPoints.

After the Execution of the command, all modifications are separately on the undo stack.

Is there way to group the transactions to one operation? To get the last state before the command by clicking Ctrl+Z once?

I´ve tried to call OpenLinkedUndo of Microsoft.VisualStudio.TextManager.Interop.VsUndoPackageClass, and CloseLinkedUndo after, but no difference.

regards