Can't reproduce HP-28 operations as objects example

[Giuseppe Donnini]

During normal operation, the active entry mode is "immediate-execution", in which several keys, among which the [+] key, perform an implicit ENTER. Otherwise, you would have to press [+], followed by an explicit [ENTER], even in very trivial calculations like 1 2 +. Besides being unnecessarily convoluted, such an entry system would also break compatibility with all previous RPN calculators.

To solve your problem, just activate ALPHA mode (by pressing [α]) before pressing [+]. This will simply add the "+" to the text already present in the command line, without executing the latter.