This interface declares similar methods as the commons logging API.
Used if no log facade is found.
Allows to choose between the different handled log factories : slf4j or commons logging, depending on what is available on CLASS_PATH.
Redirects to commons logging facade.
Redirects to slf4j facade, which does not recognize the "fatal" level.
Holds reference to the Marker used for FATAL log level, with label "FATAL", in order to avoid initialization of the marker if class is loaded and slf is not in the classpath.