nsresult ChildAt(PRInt32 aIndex, nsIContent*& aResult) const { aResult = nsnull; return NS_OK; }
nsGenericHTMLContainerElement::ChildAt(PRInt32 aIndex, nsIContent** aResult) const {   nsIContent *child = (nsIContent *)mChildren.SafeElementAt(aIndex); NS_IF_ADDREF(child); *aResult = child;  return NS_OK; }
  nsresult ChildAt(PRInt32 aIndex, nsIContent*& aResult) const {    aResult = nsnull;    return NS_OK;  }