### Question:
DocumentSet implements Iterable<Document> { public int size() { return keyIndex.size(); } private  DocumentSet(
      ImmutableSortedMap<DocumentKey, Document> keyIndex, ImmutableSortedSet<Document> sortedSet); static DocumentSet emptySet(final Comparator<Document> comparator); int size(); boolean isEmpty(); boolean contains(DocumentKey key); @Nullable Document getDocument(DocumentKey key); @Nullable Document getFirstDocument(); @Nullable Document getLastDocument(); @Nullable Document getPredecessor(DocumentKey key); int indexOf(DocumentKey key); DocumentSet add(Document document); DocumentSet remove(DocumentKey key); List<Document> toList(); @Override @NonNull Iterator<Document> iterator(); @Override boolean equals(Object other); @Override int hashCode(); @Override String toString();  }### Answer:
@Test public void testCount() { assertEquals(0, docSet(TEST_COMPARATOR).size()); assertEquals(3, docSet(TEST_COMPARATOR, DOC1, DOC2, DOC3).size()); }