AutoIncrement dependent on a field

[Wikunia]

It would be great if the autoincrement field could be dependent on a special field. For exampe:

<form> action template tpl:id:template "@@field@@"
textbox "field"
select   "ID-type" "aIb|c"
hiddenautoinc "id" @@ID-type@@
submit "submit" </form>

This should mean, that there is a hiddenautoinc for each ID-type.

For example: I want to create a new document with an ID-type "a" then in "id" should be a 0. If I want to add afterwards a new document with an ID-type "b" then in "id" should be a 0, again, because of three different hiddenautoinc fields (one for each ID-type)