// Test #keys()
    assertMultisetIsUnmodifiable(multimap.keys(), sampleKey);
    assertMultimapRemainsUnmodified(multimap, originalEntries);

    // Test #keySet()
    assertSetIsUnmodifiable(multimap.keySet(), sampleKey);
    assertMultimapRemainsUnmodified(multimap, originalEntries);

    // Test #get()
    if (!multimap.isEmpty()) {
      K key = multimap.keySet().iterator().next();

	// preceding the original header. This file contains a set of key-value
	// records, which are used to overcome USTAR's shortcomings, in addition to
	// providing the ability to have sub-second resolution for timestamps.
	//
	// Some newer formats add their own extensions to PAX by defining their
	// own keys and assigning certain semantic meaning to the associated values.

        }
        return asMap;
      }
    }

    @Override
    public Multiset<K> keys() {
      synchronized (mutex) {
        if (keys == null) {
          keys = multiset(delegate().keys(), mutex);
        }
        return keys;
      }
    }

    @Override
    // A forwarding implementation can't do any better than the underlying object.

        }
        return asMap;
      }
    }

    @Override
    public Multiset<K> keys() {
      synchronized (mutex) {
        if (keys == null) {
          keys = multiset(delegate().keys(), mutex);
        }
        return keys;
      }
    }

    @Override
    // A forwarding implementation can't do any better than the underlying object.

#### Rationale

- SSE-C requires application to remember the keys for all GET/PUT operations, any unfortunate loss of keys would automatically mean the objects cannot be accessed anymore.
- SSE-C is hardly adopted by most widely used applications, applications prefer server to manage the keys via SSE-KMS or SSE-S3.

    }
    assertFailsWith<IllegalArgumentException> {
      key = "has_LF\n"
      cache.edit(key)
    }.also { expected ->
      assertThat(expected.message).isEqualTo("keys must match regex [a-z0-9_-]{1,120}: \"$key\"")
    }
    assertFailsWith<IllegalArgumentException> {
      key = "has_invalid/"
      cache.edit(key)
    }.also { expected ->

   * Multimap#asMap} view determines the ordering in which data is written to the stream.
   *
   * <p>The serialized output consists of the number of distinct keys, and then for each distinct
   * key: the key, the number of values for that key, and the key's values.
   */
  static <K extends @Nullable Object, V extends @Nullable Object> void writeMultimap(
      Multimap<K, V> multimap, ObjectOutputStream stream) throws IOException {

var (
	DefaultKVS = config.KVS{
		config.KV{
			Key:   Endpoints,
			Value: "",
		},
		config.KV{
			Key:   PathPrefix,
			Value: "",
		},
		config.KV{
			Key:   CoreDNSPath,
			Value: "/skydns",
		},
		config.KV{
			Key:   ClientCert,
			Value: "",
		},
		config.KV{
			Key:   ClientCertKey,
			Value: "",
		},
	}
)

// Config - server etcd config.

   * algorithms should document their behavior in the presence of duplicates.
   *
   * <p>The elements of the returned {@code Iterable} must each be:
   *
   * <ul>
   *   <li>Non-null
   *   <li>Usable as {@code Map} keys (see the Guava User Guide's section on <a
   *       href="https://github.com/google/guava/wiki/GraphsExplained#graph-elements-nodes-and-edges">
   *       graph elements</a> for details)
   * </ul>
   *

 * @since 20.0
 */
@Beta
public interface MutableNetwork<N, E> extends Network<N, E> {

  /**
   * Adds {@code node} if it is not already present.
   *
   * <p><b>Nodes must be unique</b>, just as {@code Map} keys must be. They must also be non-null.
   *
   * @return {@code true} if the network was modified as a result of this call
   */
  @CanIgnoreReturnValue
  boolean addNode(N node);

  /**