XSLT Function that creates a unique reference string for each TEI element

Question

I have a in TEI with multiple elements. Basically I want to transform the list from this

<listEvent>
<event> <!--other stuff here--> </event>
<event> <!--other stuff here--> </event>
<event> <!--other stuff here--> </event>
</listEvent>

to

<listEvent>
<event ana="event1"> <!--other stuff here--> </event>
<event ana="event2"> <!--other stuff here--> </event>
<event ana="event3"> <!--other stuff here--> </event>
</listEvent>

and so on. So every is supposed to have a @ana Attribute with a unique value.

i don't know how to do this.