@CheckReturnValue
    default public LoggingEventBuilder atWarn() {
        if (isWarnEnabled()) {
            return makeLoggingEventBuilder(WARN);
        } else {
            return NOPLoggingEventBuilder.singleton();
        }
    }
Entry point for fluent-logging for {@link org.slf4j.event.Level#WARN} level. 
 
@return LoggingEventBuilder instance as appropriate for level WARN
@since 2.0
slf4j-api/src/main/java/org/slf4j/Logger.java