code: public Set<Map.Entry<String, CacheEntry>> entrySet() {
  		Set<Map.Entry<String, CacheEntry>> result = Collections.emptySet();
  		NavigableMap<String, CacheEntry> navMap = new TreeMap<String, CacheEntry>(map);
  		String from = navMap.ceilingKey(keyPrefix), to = navMap.lowerKey(keyPrefixUpperBound);
  		if (from != null) {
  			result = navMap.subMap(from, true, to, true).entrySet();
  		}
  		return result;
  	}
docs: Like {@link Map#entrySet()} except that the returned entrySet
  contains only entries belonging to this layer
  
  @return entry set of map entries belonging to this layer