    unsigned int& released_key(const unsigned int pos=0) const {
      static unsigned int key0;
      return pos<128?_released_keys[pos]:(key0 = 0);
    }
    unsigned int& released_key(const unsigned int pos=0) const {
      static unsigned int key0;
      return pos<128?_released_keys[pos]:(key0 = 0);
    }