docs: public final void mergeActiveKeyChains(KeyChainGroup from, long keyRotationTimeSecs) {
          checkArgument(isEncrypted() == from.isEncrypted(), "encrypted and non-encrypted keychains cannot be mixed");
          for (DeterministicKeyChain chain : from.getActiveKeyChains(keyRotationTimeSecs))
              addAndActivateHDChain(chain);
      }