return tailMap(fromKey, true);
  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return new Maps.NavigableKeySet<>(this);
  }

  @Override
  public Set<K> keySet() {
    return navigableKeySet();
  }

  @Override
  public NavigableSet<K> descendingKeySet() {
    return descendingMap().navigableKeySet();
  }

  @Override
  public NavigableMap<K, V> descendingMap() {

      };
    }
  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return delegate().navigableKeySet();
  }

  /**
   * A sensible implementation of {@link NavigableMap#navigableKeySet} in terms of the methods of
   * this {@code NavigableMap}. In many cases, you may wish to override {@link
   * ForwardingNavigableMap#navigableKeySet} to forward to this implementation or a subclass
   * thereof.
   *

    return tailMap(fromKey, true);
  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return new Maps.NavigableKeySet<>(this);
  }

  @Override
  public Set<K> keySet() {
    return navigableKeySet();
  }

  @Override
  public NavigableSet<K> descendingKeySet() {
    return descendingMap().navigableKeySet();
  }

  @Override
  public NavigableMap<K, V> descendingMap() {

      };
    }
  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return delegate().navigableKeySet();
  }

  /**
   * A sensible implementation of {@link NavigableMap#navigableKeySet} in terms of the methods of
   * this {@code NavigableMap}. In many cases, you may wish to override {@link
   * ForwardingNavigableMap#navigableKeySet} to forward to this implementation or a subclass
   * thereof.
   *

  }

  public void testNavigableKeySet() {
    NavigableMap<String, Integer> map = create();
    NavigableSet<String> navigableKeySet = map.navigableKeySet();
    assertTrue(navigableKeySet instanceof SynchronizedNavigableSet);
    assertThat(((SynchronizedNavigableSet<String>) navigableKeySet).mutex).isSameInstanceAs(mutex);
  }

  public void testPollFirstEntry() {
    create().pollFirstEntry();
  }

    }

    @Override
    NavigableSet<K> createKeySet() {
      return new NavigableKeySet(sortedMap());
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
      return keySet();
    }

    @Override
    public NavigableSet<K> descendingKeySet() {
      return descendingMap().navigableKeySet();
    }

    @Override
    public NavigableMap<K, Collection<V>> subMap(

  }

  @Override
  public @Nullable K lowerKey(K key) {
    return delegate.lowerKey(checkValid(key));
  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return delegate.navigableKeySet();
  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return delegate.pollFirstEntry();
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {

    }

    @Override
    NavigableSet<K> createKeySet() {
      return new NavigableKeySet(sortedMap());
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
      return keySet();
    }

    @Override
    public NavigableSet<K> descendingKeySet() {
      return descendingMap().navigableKeySet();
    }

    @Override
    public NavigableMap<K, Collection<V>> subMap(

  }

  @Override
  public @Nullable K lowerKey(K key) {
    return delegate.lowerKey(checkValid(key));
  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return delegate.navigableKeySet();
  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return delegate.pollFirstEntry();
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {

      /*
       * We can't use StandardKeySet, as NavigableMapTestSuiteBuilder assumes that our keySet is a
       * NavigableSet. We test StandardKeySet in the superclass, so it's still covered.
       */
      return navigableKeySet();
    }

    @Override
    public Collection<V> values() {
      return new StandardValues();
    }

    @Override
    public String toString() {
      return standardToString();
    }