HP HP-28S - If You Execute the Wrong Function

If You Execute the Wrong Function
The HP-28S includes recovery features to help you "backtrack" when
you mistakenly execute a function. The following steps reverse the
effects of a function, whether a one-number or two-number function.
1. Press .1 UNDO 1 to recover the previous contents of the stack.
2. If a number was in the command line when you made the mis-
take, press
.1 COMMAND I to recover the previous contents of the
command line.
3. Continue the calculation.
2: Doing Arithmetic 47