* the methods of the delegate. For example, overriding {@link #getValue} alone <i>will not</i>
 * change the behavior of {@link #equals}, which can lead to unexpected behavior. In this case, you
 * should override {@code equals} as well, either providing your own implementation, or delegating
 * to the provided {@code standardEquals} method.
 *

 * <p><b>Warning:</b> The methods of {@code ForwardingMap} forward <i>indiscriminately</i> to the
 * methods of the delegate. For example, overriding {@link #put} alone <i>will not</i> change the
 * behavior of {@link #putAll}, which can lead to unexpected behavior. In this case, you should
 * override {@code putAll} as well, either providing your own implementation, or delegating to the
 * provided {@code standardPutAll} method.
 *

 * <b>indiscriminately</b> to the methods of the delegate. For example, overriding {@link
 * #add(Object, int)} alone <b>will not</b> change the behavior of {@link #add(Object)}, which can
 * lead to unexpected behavior. In this case, you should override {@code add(Object)} as well,
 * either providing your own implementation, or delegating to the provided {@code standardAdd}
 * method.
 *

 * the methods of the delegate. For example, overriding {@link #getValue} alone <i>will not</i>
 * change the behavior of {@link #equals}, which can lead to unexpected behavior. In this case, you
 * should override {@code equals} as well, either providing your own implementation, or delegating
 * to the provided {@code standardEquals} method.
 *

 * the methods of the delegate. For example, overriding {@link #add(Object, int)} alone <b>will
 * not</b> change the behavior of {@link #add(Object)}, which can lead to unexpected behavior. In
 * this case, you should override {@code add(Object)} as well, either providing your own
 * implementation, or delegating to the provided {@code standardAdd} method.
 *

   * message must mention two values that were associated with the duplicate key in two different
   * calls to Builder.put (though we don't really care *which* two values if there were more than
   * two). These considerations lead us to have a field of type DuplicateKey in the Builder, which
   * will remember the first duplicate key we encountered. All later calls to buildOrThrow() can

 * <b>indiscriminately</b> to the methods of the delegate. For example, overriding {@link
 * #add(Object, int)} alone <b>will not</b> change the behavior of {@link #add(Object)}, which can
 * lead to unexpected behavior. In this case, you should override {@code add(Object)} as well,
 * either providing your own implementation, or delegating to the provided {@code standardAdd}
 * method.
 *

   * higher value than you need can waste space and time, and a significantly lower value can lead
   * to thread contention. But overestimates and underestimates within an order of magnitude do not
   * usually have much noticeable impact. A value of one permits only one thread to modify the map
   * at a time, but since read operations can proceed concurrently, this still yields higher

    Node<K, V> head;
    Node<K, V> tail;
    int count;

    KeyList(Node<K, V> firstNode) {
      this.head = firstNode;
      this.tail = firstNode;
      firstNode.previousSibling = null;
      firstNode.nextSibling = null;
      this.count = 1;
    }
  }

  private transient @Nullable Node<K, V> head; // the head for all keys

 * read-write locks. Conceptually, lock striping is the technique of dividing a lock into many
 * <i>stripes</i>, increasing the granularity of a single lock and allowing independent operations
 * to lock different stripes and proceed concurrently, instead of creating contention for a single
 * lock.
 *
 * <p>The guarantee provided by this class is that equal keys lead to the same lock (or semaphore),