IF tf instructionlist
(IF tf instructionlist1 instructionlist2)

	command.  If the first input has the value TRUE, then IF runs
	the second input.  If the first input has the value FALSE, then
	IF does nothing.  (If given a third input, IF acts like IFELSE,
	as described below.)  It is an error if the first input is not
	either TRUE or FALSE.

	For compatibility with earlier versions of Logo, if an IF
	instruction is not enclosed in parentheses, but the first thing
	on the instruction line after the second input expression is a
	literal list (i.e., a list in square brackets), the IF is
	treated as if it were IFELSE, but a warning message is given.
	If this aberrant IF appears in a procedure body, the warning is
	given only the first time the procedure is invoked in each Logo
	session.