HP -28S Manual page 205

Numerical Result Mode. This alternative is used in plotting and by
the Solver. Its purpose is to ensure a numerical result from the func-
tion.
In
this mode, functions repeatedly evaluate symbolic arguments,
accepting only numerical arguments and returning numerical results.
You can force evaluation of an object until it returns a numerical re-
sult by executing -NUM (to number); in chapter 5 you did this to
return a numerical value for
7r.
U .. -
"
Note
In
Numerical Result mode, do not evaluate a variable
whose value includes its own name, such as a variable
named X that contains the expression 'X
+
Y. Evaluating
such a variable causes an endless loop. To halt an end-
less loop, you must perform a system halt (I
ON
I~),
which also clears
the stack.
Similarly, do not create variables that reference one another in a cir-
cular definition. Evaluating a variable included in a circular definition
also causes an endless loop.
204 23: Evaluation