Lua primitives: log_message()
If lumail is compiled with debugging support enabled then output will be written to a logfile. (The logfile name is specified via the --debug command-line flag.)
The log_message() primitive will allow you to append your own message to that logfile.
log_message( "The user is " .. os.getenv( "USER" ) );
This function has been available since version 0.10.