Bugger, @LN bet me to it... but here's my take! 
categories.xls (9.5 KB)
Ha, never under-estimate what cleverness you can put into an XForm, with a bit of creativity... 
[alas, I must rescind your 'solution' @Grzesiek2010 and give it to a well-deserving @LN, but I'm sure you'll earn it back soon enough 
]