docs: public DeterministicKey currentKey(KeyChain.KeyPurpose purpose) {
          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");
          }
          DeterministicKey current = currentKeys.get(purpose);
          if (current == null) {
              current = freshKey(purpose);
              currentKeys.put(purpose, current);
          }
          return current;
      }