code: List<Message> getMessages(final Level level) {
          if (level == null) {
              throw new NullPointerException("Level can't be null");
          }
  
          return Collections.unmodifiableList(this.messages.get(level));
      }
docs: Returns an inmutable copy of the {@link Message}s contained in a {@link Level}
  
  @param level
  @return