code: public static Span findWithin(List<Repeater<?>> tags, Span span, Pointer.PointerType pointer, Options options) {
      if (options.isDebug()) {
        System.out.println("Chronic.findWithin: " + tags + " in " + span);
      }
      if (tags.isEmpty()) {
        return span;
      }
      Repeater<?> head = tags.get(0);
      List<Repeater<?>> rest = (tags.size() > 1) ? tags.subList(1, tags.size()) : new LinkedList<Repeater<?>>();
      head.setStart((pointer == Pointer.PointerType.FUTURE) ? span.getBeginCalendar() : span.getEndCalendar());
      Span h = head.thisSpan(PointerType.NONE);
  
      if (span.contains(h.getBegin()) || span.contains(h.getEnd())) {
        return findWithin(rest, h, pointer, options);
      }
      return null;
    }
docs: Recursively finds repeaters within other repeaters.
  Returns a Span representing the innermost time span
  or nil if no repeater union could be found