7D. built-in constants
Following are built-in constants that may be used:

TRUE,FALSE	Represent the boolean values (-1,0)
NIL		(=0), the uninitialised pointer.
ALL		Used with string functions like StrCopy() to copy all characters
GADGETSIZE	Minimum size in bytes to hold one gadget (see  9D , Gadget()) [obsolete]
OLDFILE,NEWFILE	Mode-parameters for use with Open()
EMPTY		used with methods (might be keyword in the future)
STRLEN		Always has the value of the length of the last immediate
		string used. Example:

		Write(handle,'hi folks!',STRLEN)      /* =9 */