NS_ASSERTION(mStackPos > 0, "leaf w/o container");
if (mStackPos <= 0) { return NS_ERROR_FAILURE; }
SinkContext::AddLeaf(nsIHTMLContent* aContent){  NS_ASSERTION(mStackPos > 0, "leaf w/o container");  nsIHTMLContent* parent = mStack[mStackPos-1].mContent;  // If the parent has an insertion point, insert rather than  // append.  if (mStack[mStackPos-1].mInsertionPoint != -1) {    parent->InsertChildAt(aContent,                           mStack[mStackPos-1].mInsertionPoint++,                           mSink->IsInScript());  }  else {    parent->AppendChildTo(aContent, mSink->IsInScript());  }  DidAddContent(aContent, mSink->IsInScript());#ifdef DEBUG  if (mPreAppend &&       SINK_LOG_TEST(gSinkLogModuleInfo,SINK_ALWAYS_REFLOW)) {    mSink->ForceReflow();  }#endif  return NS_OK;}