Count books in all book categories

Hello experts,

I would like to count the number of books in each book category. I have an
data example here:

<bookshelf>
  <book><cat>A</cat></book>  
  <book><cat>B</cat></book>
  <book><cat>A</cat></book>
  <book><cat>B</cat></book>
  <book><cat>B</cat></book>
</bookself>

The result should be:

A    2
B    3

It would be nice if the script would recognise the different categories by
itself (e.g. starting a new one if it has not be seen before). But to make
things simple it would also be ok to have one line for each category (since
I know them and there are fix).

Can somebody help me?

Kind Regards,
Karl


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