code: private OriginalMapping getPreviousMapping(int lineNumber) {
      do {
        if (lineNumber == 0) {
          return null;
        }
        lineNumber--;
      } while (lines.get(lineNumber) == null);
      ArrayList<Entry> entries = lines.get(lineNumber);
      return getOriginalMappingForEntry(Iterables.getLast(entries));
    }
docs: Returns the mapping entry that proceeds the supplied line or null if no
  such entry exists.