Update facility

I have written a function consisting in a copy/modify expression:

declare function local:mine($s)
{
copy $e:= $s
modify (...)
return $e
};

It seems it is not possible to somehow make it recursive ( ... return local:mine($e) ): can anyone confirm this? What is the best way to have a recursive function while using the update facility? Best, Giuseppe

Could you please give us a full example (input, expected output?