NS_IMETHOD ChildAt(PRInt32 aIndex, nsIContent*& aResult) const { aResult = nsnull; return NS_OK;  }
NS_IMETHOD ChildAt(PRInt32 aIndex, nsIContent** aResult) const { aResult = nsnull; return NS_OK;  }
  NS_IMETHOD ChildAt(PRInt32 aIndex, nsIContent*& aResult) const { aResult = nsnull; return NS_OK;  }