Function: _def_log
Class: default
Section: default
C-Name: sd_log
Prototype:
Help:
Doc: this can be either 0 (off) or 1, 2, 3
 (on, see below for the various modes). When logging mode is turned on, \kbd{gp}
 opens a log file, whose exact name is determined by the \kbd{logfile}
 default. Subsequently, all the commands and results will be written to that
 file (see \b{l}). In case a file with this precise name already existed, it
 will not be erased: your data will be \emph{appended} at the end.

 The specific positive values of \kbd{log} have the following meaning

 1: plain logfile

 2: emit color codes to the logfile (if \kbd{colors} is set).

 3: write LaTeX output to the logfile (can be further customized using
 \tet{TeXstyle}).

 The default value is \kbd{0}.

 \misctitle{Note} Logging starts as soon as \kbd{log} is set to a nonzero
 value. In particular, when \kbd{log} is set in \kbd{gprc}, warnings and
 errors triggered from the rest of the file will be written in the logfile.
 For instance, on clean startup, the logfile will start by \kbd{Done.}
 (from the \kbd{Reading GPRC:\dots Done.} diagnostic printed when starting
 \kbd{gp}), then the \kbd{gp} header and prompt.
