return NS_ERROR_NULL_POINTER;
return NS_ERROR_FAILURE;
nsSessionHistory::GoBack(nsIWebShell * prev){   nsresult rv = NS_OK;   //nsHistoryEntry * hEntry=nsnull;   if (mHistoryCurrentIndex <= 0)       return NS_ERROR_NULL_POINTER;   Goto((mHistoryCurrentIndex -1), prev);   return NS_OK;}