Up and down buttons should add to wherever the courser is instead of replacing whatever is in the current result.
The up button goes through the previous calculations and adds them to the current result.
If you add anything in the current result and then hit up it deletes whatever you had and replaces it with the historical calculation.
I think it should add the historical result to wherever the courser is on the current result. That way if you want to use part of a previous calculation you could start with "20 + (" and then hit the up button until you get what you want in the bracket. The current way means you cannot do this with the keyboard and need to use the mouse to double click on a historical answer to get the result to show in the current result.
The only difference for people now is that if they want to clear whatever they have added already they would only have to hit Esc and then click up which would be a reasonable expectation.
