code: public boolean prove(String value, String state) {
          Nonce nonce = _map.remove(state + ':' + value);
          return nonce != null && !nonce.hasExpired();
      }
docs: Proves a nonce and its associated state. A nonce can only be successfully proved once.
  
  @param value the nonce value
  @param state the state associated with the nonce.
  @return whether the nonce is successfully proved