Reserved REXX Variables

What REXX variables are already defined? Which variables should you not use for any other purpose?

 Reserved REXX Variables

REXX has three variables with a predefined meaning. Using these for anything but their original purpose will cause 10% of your brain cells to deteriorate...
RC
 is the numeric return code of any operating system or environment command.
	"LISTCAT"
	SAY RC
A zero means that there were no problems. A positive number indicates some problem. 
A negative number tells of a serious problem. 
REXX does not understand these codes. 
The operating system command manual will explain them.

SIGL is the line number of the source program that sent control to a subroutine, internal function, or condition trap.
	ERROR:
	Say "Line #" SIGL "sent control to this subroutine"

RESULT is the character string or number returned by a function (built-in or user-written) when it is invoked with a CALL.
User-written functions must set this by placing a character string or number on their RETURN statement. Built-in functions do it automatically. 
	/* in the main part of your program*/
	CALL Length "ABCDEF"

	Say RESULT


	/* or*/
	CALL My_function
		/*reminder - RESULT is set only after a CALL*/
	Say RESULT

	EXIT

	My_function:
	/* does some processing */
	RETURN /*character string or number goes here*/