+\verbatim --log=root.thres:debug --log=root.thres:critical\endverbatim
+
+\subsection log_use_conf_fmt 3.1.3 Format configuration
+
+As with SimGrid 3.3, it is possible to control the format of log
+messages. This is done through the <tt>fmt</tt> keyword. For example,
+\verbatim --log=root.fmt:%m\endverbatim reduces the output to the
+user-message only, removing any decoration such as the date, or the
+process ID, everything.
+
+Here are the existing format directives:
+
+ - %%: the % char
+ - %%n: platform-dependant line separator (LOG4J compliant)
+ - %%e: plain old space (SimGrid extension)
+
+ - %%m: user-provided message
+
+ - %%c: Category name (LOG4J compliant)
+ - %%p: Priority name (LOG4J compliant)
+
+ - %%h: Hostname (SimGrid extension)
+ - %%P: Process name (SimGrid extension)
+ - %%t: Thread "name" (LOG4J compliant -- actually the address of the thread in memory)
+ - %%i: Process PID (SimGrid extension -- this is a 'i' as in 'i'dea)
+
+ - %%F: file name where the log event was raised (LOG4J compliant)
+ - %%l: location where the log event was raised (LOG4J compliant, like '%%F:%%L' -- this is a l as in 'l'etter)
+ - %%L: line number where the log event was raised (LOG4J compliant)
+ - %%M: function name (LOG4J compliant -- called method name here of course).
+ Defined only when using gcc because there is no __FUNCTION__ elsewhere.
+
+ - %%b: full backtrace (Called %%throwable in LOG4J).
+ Defined only when using the GNU libc because backtrace() is not defined
+ elsewhere.
+ - %%B: short backtrace (only the first line of the %%b).
+ Called %%throwable{short} in LOG4J; defined where %%b is.
+
+ - %%d: date (UNIX-like epoch)
+ - %%r: application age (time elapsed since the beginning of the application)
+
+
+If you want to mimick the simple layout with the format one, you would use this
+format: '[%%h:%%i:(%%I) %%r] %%l: [%%c/%%p] %%m%%n'. This is not completely correct
+because the simple layout do not display the message location for messages at
+priority INFO (thus, the fmt is '[%%h:%%i:(%%I) %%r] %%l: [%%c/%%p] %%m%%n' in this
+case). Moreover, if there is no process name (ie, messages comming from the
+library itself, or test programs doing strange things) do not display the
+process identity (thus, fmt is '[%%r] %%l: [%%c/%%p] %%m%%n' in that case, and '[%%r]
+[%%c/%%p] %%m%%n' if they are at priority INFO).
+
+For now, there is only one format modifyier: the precision field. You
+can for example specify %.4r to get the application age with 4
+numbers after the radix. Another limitation is that you cannot set
+specific layouts to the several priorities.
+
+\subsection log_use_conf_add 3.1.4 Category additivity
+
+The <tt>add</tt> keyword allows to specify the additivity of a
+category (see \ref log_in_app). This is rarely useful since you
+cannot specify an alternative appender. Anyway, '0', '1', 'no',
+'yes', 'on' and 'off' are all valid values, with 'yes' as default.