docs: public List<DeterministicKey> freshKeys(KeyChain.KeyPurpose purpose, int numberOfKeys) {
          DeterministicKeyChain chain = getActiveKeyChain();
          if (chain.isMarried()) {
              throw new UnsupportedOperationException("Key is not suitable to receive coins for married keychains." +
                      " Use freshAddress to get P2SH address instead");
          }
          return chain.getKeys(purpose, numberOfKeys);   // Always returns the next key along the key chain.
      }