Iterator<? extends T> iterator = source.iterator();
        if (!iterator.hasNext()) {
            throw new NoSuchElementException("Expecting collection with single element, got none.");
        }
        T element = iterator.next();
        if (iterator.hasNext()) {
            throw new IllegalArgumentException("Expecting collection with single element, got multiple.");
        }

  public <A extends Appendable> A appendTo(A appendable, Iterator<? extends @Nullable Object> parts)
      throws IOException {
    checkNotNull(appendable);
    if (parts.hasNext()) {
      appendable.append(toString(parts.next()));
      while (parts.hasNext()) {
        appendable.append(separator);
        appendable.append(toString(parts.next()));
      }
    }
    return appendable;
  }

  /**

                public Iterator<Entry<Object, Object>> iterator() {
                    return new Iterator<Entry<Object, Object>>() {
                        @Override
                        public boolean hasNext() {
                            return false;
                        }

                        @Override
                        public Entry<Object, Object> next() {

      Iterable<? extends T> iterable, Comparator<T> comparator) {
    checkNotNull(comparator);
    Iterator<? extends T> it = iterable.iterator();
    if (it.hasNext()) {
      T prev = it.next();
      while (it.hasNext()) {
        T next = it.next();
        if (comparator.compare(prev, next) > 0) {
          return false;
        }
        prev = next;
      }
    }
    return true;
  }

    int i = 0;
    while (expectedIterator.hasNext()) {
      Object expected = expectedIterator.next();

      assertTrue(
          "index " + i + " expected <" + expected + "., actual is exhausted",
          actualIterator.hasNext());

      Object actual = actualIterator.next();
      assertEquals("index " + i, expected, actual);
      i++;
    }
    if (actualIterator.hasNext()) {

                                builder.keyRing(keyring);
                                Iterator<PGPPublicKey> pkIt = keyring.getPublicKeys();
                                while (pkIt.hasNext()) {
                                    PGPPublicKey publicKey = pkIt.next();
                                    Fingerprint fingerprint = Fingerprint.of(publicKey);

        Iterator<C> elemItr = Iterators.emptyIterator();

        @Override
        @CheckForNull
        protected C computeNext() {
          while (!elemItr.hasNext()) {
            if (rangeItr.hasNext()) {
              elemItr = ContiguousSet.create(rangeItr.next(), domain).iterator();
            } else {
              return endOfData();
            }
          }

            boolean expectConflictResolution
            int idx = 0

            @Override
            boolean hasNext() {
                batchesIterator.hasNext() || current?.hasNext()
            }

            @Override
            Candidate next() {
                if (current?.hasNext()) {
                    if (expectedSingle instanceof Iterable) {
                        expectedSingle = expectedSingle.iterator()

    while (keyValues.hasNext() && newValues.hasNext()) {
      keyValues.next();
      keyValues.set(newValues.next());
    }

    // Remove remaining old values, if any.
    while (keyValues.hasNext()) {
      keyValues.next();
      keyValues.remove();
    }

    // Add remaining new values, if any.
    while (newValues.hasNext()) {
      keyValues.add(newValues.next());
    }

        Iterator<C> elemItr = Iterators.emptyIterator();

        @Override
        @CheckForNull
        protected C computeNext() {
          while (!elemItr.hasNext()) {
            if (rangeItr.hasNext()) {
              elemItr = ContiguousSet.create(rangeItr.next(), domain).iterator();
            } else {
              return endOfData();
            }
          }