code: public T watchAndFollow(DeterministicKey accountKey) {
              checkState(accountPath == null, "either watchAndFollow or accountPath");
              this.watchingKey = accountKey;
              this.isFollowing = true;
              return self();
          }
docs: Creates a deterministic key chain with the given watch key and that follows some other keychain. In a married
  wallet following keychain represents "spouse". Watch key has to be an account key.