internal void CloneFrom(PDFDocumentCitationManager other)
        {
            // prevent deadlock due to possible incorrect use of this API:
            if (other != this)
            {
                // Utilities.LockPerfTimer l1_clk = Utilities.LockPerfChecker.Start();
                lock (citations_lock)
                {
                    // l1_clk.LockPerfTimerStop();

                    // the other PDFCitMgr instance has its own lock, so without
                    // a copy through `other.Citations_LOCKED` this code would
                    // be very much thread-UNSAFE!
                    Citations_RAW.AddRange(other.Citations_LOCKED);
                    WriteToDisk(pdf_document, Citations_RAW);
                }
            }
        }
 internal void CloneFrom(PDFDocumentCitationManager other)
 {
     this.Citations.AddRange(other.Citations);
     WriteToDisk(pdf_document, Citations);
 }