log.go
func (l *slog) Criticalf(format string, args ...interface{}) {
	lvl := l.Level()
	if lvl <= LevelCritical {
		l.b.printf("CRT", l.tag, format, args...)
	}
}
func (l *slog) Criticalf(format string, args ...interface{}) {
	lvl := l.Level()
	if lvl <= LevelCritical {
		l.b.printf("CRT", l.tag, format, args...)
	}
}
// Criticalf formats message according to format specifier, prepends the prefix
// as necessary, and writes to log with LevelCritical.
//
// This is part of the Logger interface implementation.