This "goal", with no arguments, allows you to exit from Prolog
in an operating-system-independent way. Typing the "end-of-file"
character (control-D in Unix/Linux systems) will also get you out.
Different operating systems may use different end-of-file
characters. Example of
--- Welcome message from Prolog interpreter ---
%" signs in this example represent the operating system
command interpreter (aka "shell") prompt, not a Prolog