Need help with special sorting

Hi,

I need some help with 'special sorting'.
I have this xml-file (simplified).

<alfabet>
        <letter>a</letter>
        <letter>b</letter>
        <letter>c</letter>
        <letter>d</letter>
</alfabet>

The outcome after sorting should however be this:
a b d c

The letter c has always to be the last item after sorting.
How can I achieve this?

Kind regards,
Bert