recursion

Question

Hello,
 just a question; I need to do recurion on a sort of tree but I need to use 'yield' keyword as well. But recursion it seems don't work:
public IEnumerable compute(string query, string s)  {
       foreach (string s in Coll) {
          query = .........................;         
          if () yield return compute(query, s);
       }      
}
This is a pseudo-code of course; I wonder if womething wrong in what I'm trying to do...
thanks