Section 3: Automatic Memory Stack and ALPHA Register41
Try using APPEND now:
Keystrokes Display    
ADD  ADD _   The initial string.
ALPHA   0.2000   Takes the HP-41C out of ALPHA mode.
ALPHA   ADD   Places the HP-41C back into ALPHA mode.
g APPEND ( g K)  ADD _   This enables you to continue adding characters to the string into ALPHA register.
ITION  ADDITION   The entire string.
ALPHA   0.2000    
If you don’t press APPEND before adding new characters, the new characters will clear the previous string from the ALPHA register. For example:
Keystrokes Display    
ALPHA   ADDITION   The old string.
RUN  RUN _   The new string clears the previous string from the ALPHA register.
ALPHA   0.2000    
Function Names and the Display
Each time you press and hold a function key for a moment, a name describing that function appears in the display. When you release the key, the name goes away and the function is executed.
If you press and hold a function key for longer than about a half second, the name will appear, and then will be replaced by the word NULL. NULL indicates that the function has been nullified and will not be executed when you release the key. This allows you to preview function names and quickly recover from keystroke errors.
Keys that you use to key in numbers ( CHS , EEX , · and 0 through 9) and ALPHA characters do not prompt with a name in the display. These keys execute when the key is pressed—they cannot be nullified.
Clearing the ALPHA- and X-registers
You can clear the contents of the ALPHA register in ALPHA mode by pressing g CLA. CLA (clear ALPHA) clears the ALPHA register and leaves the automatic memory stack intact.