NS_ASSERTION(oldLastChild != sib, "Loop in frame list");
nsFrameItems::AddChild(nsIFrame* aChild){  if (childList == nsnull) {    childList = lastChild = aChild;  }  else  {    lastChild->SetNextSibling(aChild);    lastChild = aChild;  }  // if aChild has siblings, lastChild needs to be the last one  for (nsIFrame* sib = lastChild->GetNextSibling(); sib;       sib = sib->GetNextSibling()) {    lastChild = sib;  }}