code: private void genFallbackPattern(int field, String skeleton,
                                      Map<String, PatternInfo> intervalPatterns,
                                      DateTimePatternGenerator dtpng) {
          String pattern = dtpng.getBestPattern(skeleton);
          // for fall back interval patterns,
          // the first part of the pattern is empty,
          // the second part of the pattern is the full-pattern
          // should be used in fall-back.
          PatternInfo ptn = new PatternInfo(
                                      null, pattern, fInfo.getDefaultOrder());
          intervalPatterns.put( 
              DateIntervalInfo.CALENDAR_FIELD_TO_PATTERN_LETTER[field], ptn);
      }
docs: /*
  Generate fall back interval pattern given a calendar field,
  a skeleton, and a date time pattern generator
  @param field      the largest different calendar field
  @param skeleton   a skeleton
  @param dtpng      date time pattern generator
  @param intervalPatterns interval patterns