unwillingly took the place of the March Hare.  The Hatter was the
only one who got any advantage from the change:  and Alice was a
good deal worse off than before, as the March Hare had just upset
the milk-jug into his plate.

  Alice did not wish to offend the Dormouse again, so she began
very cautiously:  `But I don't understand.  Where did they draw
the treacle from?'

  /** Singleton placeholder that indicates a value is being loaded. */
  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {
    return (ValueReference<K, V>) UNSET;
  }

  private enum NullEntry implements ReferenceEntry<Object, Object> {
    INSTANCE;

    @CheckForNull
    @Override

import static com.google.common.cache.LocalCache.DRAIN_THRESHOLD;
import static com.google.common.cache.LocalCache.nullEntry;
import static com.google.common.cache.LocalCache.unset;
import static com.google.common.cache.TestingCacheLoaders.identityLoader;
import static com.google.common.cache.TestingRemovalListeners.countingRemovalListener;

import static com.google.common.cache.LocalCache.DRAIN_THRESHOLD;
import static com.google.common.cache.LocalCache.nullEntry;
import static com.google.common.cache.LocalCache.unset;
import static com.google.common.cache.TestingCacheLoaders.identityLoader;
import static com.google.common.cache.TestingRemovalListeners.countingRemovalListener;

unwillingly took the place of the March Hare.  The Hatter was the
only one who got any advantage from the change:  and Alice was a
good deal worse off than before, as the March Hare had just upset
the milk-jug into his plate.

  Alice did not wish to offend the Dormouse again, so she began
very cautiously:  `But I don't understand.  Where did they draw
the treacle from?'

       */
      String key = (String) requireNonNull(e.nextElement());
      /*
       * requireNonNull is safe because the key came from propertyNames...
       *
       * ...except that it's possible for users to insert a string key with a non-string value, and
       * in that case, getProperty *will* return null.
       *
       * TODO(b/192002623): Handle that case: Either:
       *