return NS_ERROR_NULL_POINTER;
return NS_ERROR_FAILURE;
nsSessionHistory::GoForward(nsIWebShell * prev){   nsresult rv = NS_OK;   PRInt32  prevIndex = 0;   if (mHistoryCurrentIndex == mHistoryLength-1)      return NS_ERROR_NULL_POINTER;   Goto((mHistoryCurrentIndex+1), prev);   return NS_OK;}