docs: private final void writeHeartbeat()
      throws LockFile.FileSecurityException,
             LockFile.UnexpectedEndOfFileException,
             LockFile.UnexpectedFileIOException {
  
          try {
              raf.seek(MAGIC.length);
              raf.writeLong(System.currentTimeMillis());
          } catch (SecurityException ex) {
              throw new FileSecurityException(this, "writeHeartbeat", ex);
          } catch (EOFException ex) {
              throw new UnexpectedEndOfFileException(this, "writeHeartbeat", ex);
          } catch (IOException ex) {
              throw new UnexpectedFileIOException(this, "writeHeartbeat", ex);
          }
      }