code: public Mapper<K, V> tierKey() {
  	    if (!(delegate.get() instanceof TierKeyMap)) {
  	        delegate = Optional.of((Map<K, V>) TierKeyMap.from(delegate.get()));
  	    }
  		return this;
  	}
docs: Returns the {@link TierKeyMap} instance, allowed the key like "a.b[0].c.d[3]",
  and if the key class type is not {@link String} will happen nothing.
  
  @return