static int GetPrevOffset (TextDocument document, int lineNumber)
{
int startLineNumber = lineNumber - 1;
if (startLineNumber < 0)
startLineNumber = document.TextLength - 1;
var line = document.GetLinesReverseStartingAt (startLineNumber - 1).FirstOrDefault (l => l.IsBookmarked);
return line != null ? line.Offset : -1;
}