[Q] numeric entry in boa constructor

[Rolf Marvin Bøe Lindgren]

I've created a simple app in boa constructor.  I would like to add a
feature that does not seem to come with a pre-made component:

the user hits a number on the numeric keypad.  the number is displayed
in a field.  the user hits another number, the previous number is
erased and the new number appears.  

any clues?  

-- 
Rolf Lindgren                                            http://www.roffe.com/
roffe at tag.uio.no