HP -01 Owner's Manual page 96

Operational Description 94
At this point, if you press (@, the calculator would return to its initial state—all three registers
would be cleared. Also, at this point, you can press another operator and it will overwrite the
© in the F-register.
Press Registers Display
X F Y
© 2
Thus, if you pressed the wrong operator key, you don't have to clear and start over again.
Instead, simply key in the correct operator.
Press Registers Display
X F Y