BatLog.MakeSourceBuild a logger module with custom, fixed output, prefix and flags
print s logs the message s, returning unit.
As Printf.printf, only the message is printed to the logging output and prefixed with status information per the current flags and the currently set prefix.
fatal s logs the message s and then calls exit 1. This exits the program with return code 1.
fatalf allows a format string (as Printf.printf)and the arguments to that format string to build the logging message. Exits the program with return code 1.