6E. built-in system variables
-----------------------------
Following global variables are always available in your program,
they're called system variables.

arg		As discussed above, contains a pointer to a zero-terminated
		string, containing the command-line arguments. Don't use this
		variable if you wish to use ReadArgs() instead.
stdout		Contains a file-handle to the standard output (and input).
		If your program was started from the workbench, so no
		shell-output is available, WriteF() will open a
		CON: window for you and put its file handle here.
stdin		file-handle of standard input
conout		This is where that file handle is kept, and the console
		window will be automatically closed upon exit of your
		program. (see  9A , WriteF(), on how to use these two variables
		properly).
execbase,	These four variables are always provided with their
dosbase,	correct values.
gfxbase,
intuitionbase
stdrast		Pointer to standard rastport in use with your program,
		or NIL. The built-in graphics functions like Line()
		make use of this variable.
wbmessage	Contains a ptr to a message you got if you started
		from wb, else NIL. May be used as a boolean to detect
		if you started from workbench, or even check any
		arguments that were shift-selected with your icon.
		See WbArgs.e in the Src/Args dir how to
		make good use of wbmessage.