* Verifies that the {@code Set} returned by {@code nodes} has the expected mutability property
   * (see the {@code Graph} documentation for more information).
   */
  @Test
  public abstract void nodes_checkReturnedSetMutability();

  /**
   * Verifies that the {@code Set} returned by {@code adjacentNodes} has the expected mutability
   * property (see the {@code Graph} documentation for more information).
   */
  @Test

    SettableFuture<?> unused = SettableFuture.create();
    // Hack to load AbstractFuture et. al. in a new classloader so that it tries to re-read the
    // cancellation-cause system property. This allows us to test what happens if reading the
    // property is forbidden and then continue running tests normally in one jvm without resorting
    // to even crazier hacks to reset static final boolean fields.

   * changes).
   */
  @Override
  public ImmutableMap<Object, Collection<Object>> asMap() {
    return super.asMap();
  }

  private Object readResolve() {
    return INSTANCE; // preserve singleton property
  }

  private static final long serialVersionUID = 0;

   *
   * <ul>
   *   <li>{@code equivalent(x, x)} is true (<i>reflexive</i> property)
   *   <li>{@code equivalent(x, y)} and {@code equivalent(y, x)} each return the same result
   *       (<i>symmetric</i> property)
   *   <li>If {@code equivalent(x, y)} and {@code equivalent(y, z)} are both true, then {@code
   *       equivalent(x, z)} is also true (<i>transitive</i> property)
   * </ul>
   *

   *
   * <ul>
   *   <li>{@code equivalent(x, x)} is true (<i>reflexive</i> property)
   *   <li>{@code equivalent(x, y)} and {@code equivalent(y, x)} each return the same result
   *       (<i>symmetric</i> property)
   *   <li>If {@code equivalent(x, y)} and {@code equivalent(y, z)} are both true, then {@code
   *       equivalent(x, z)} is also true (<i>transitive</i> property)
   * </ul>
   *

    }
  }

  public void testToString() {
    for (StandardSystemProperty property : StandardSystemProperty.values()) {
      assertEquals(property.key() + "=" + property.value(), property.toString());
    }
  }

  public void testNoNullValues() {
    for (StandardSystemProperty property : StandardSystemProperty.values()) {
      // Even though the contract in System.getProperties() specifies that a value will exist for

  private final String key;

  StandardSystemProperty(String key) {
    this.key = key;
  }

  /** Returns the key used to look up this system property. */
  public String key() {
    return key;
  }

  /**
   * Returns the current value for this system property by delegating to {@link
   * System#getProperty(String)}.
   *
   * <p>The value returned by this method is non-null except in rare circumstances:
   *

    }
  }

  public void testToString() {
    for (StandardSystemProperty property : StandardSystemProperty.values()) {
      assertEquals(property.key() + "=" + property.value(), property.toString());
    }
  }

  public void testNoNullValues() {
    for (StandardSystemProperty property : StandardSystemProperty.values()) {
      // Even though the contract in System.getProperties() specifies that a value will exist for

 *       #contains}.
 *   <li>Terminology note: a range {@code a} is said to be the <i>maximal</i> range having property
 *       <i>P</i> if, for all ranges {@code b} also having property <i>P</i>, {@code a.encloses(b)}.
 *       Likewise, {@code a} is <i>minimal</i> when {@code b.encloses(a)} for all {@code b} having
 *       property <i>P</i>. See, for example, the definition of {@link #intersection intersection}.

    SettableFuture<?> unused = SettableFuture.create();
    // Hack to load AbstractFuture et. al. in a new classloader so that it re-reads the cancellation
    // cause system property.  This allows us to run with both settings of the property in one jvm
    // without resorting to even crazier hacks to reset static final boolean fields.
    System.setProperty("guava.concurrent.generate_cancellation_cause", "true");