docs: public boolean containsKey(List<K> key) {
      //     if(! (key instanceof Object[]))
      //       return false;
      //    Object[] o = (Object[]) key;
      GeneralizedCounter<K> next = this;
      for (int i=0; i<key.size()-1; i++) {
        next = next.conditionalizeHelper(key.get(i));
        if (next==null) return false;
      }
      return next.map.containsKey(key.get(key.size()-1));
    }