void HandleTextEditorDataDocumentFolded (object sender, FoldSegmentEventArgs e)
{
int start = e.FoldSegment.StartLine.LineNumber;
int end = e.FoldSegment.EndLine.LineNumber;
if (e.FoldSegment.IsFolded) {
if (e.FoldSegment.Marker != null)
HeightTree.Unfold (e.FoldSegment.Marker, start, end - start);
e.FoldSegment.Marker = HeightTree.Fold (start, end - start);
} else {
HeightTree.Unfold (e.FoldSegment.Marker, start, end - start);
e.FoldSegment.Marker = null;
}
}