return delegate.get(checkValid(key));
  }

  @Override
  public SortedMap<K, V> headMap(K toKey) {
    return headMap(toKey, false);
  }

  @Override
  public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
    return new SafeTreeMap<>(delegate.headMap(checkValid(toKey), inclusive));
  }

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

    }

    @Override
    public SortedMap<R, Map<C, V>> headMap(R toKey) {
      checkNotNull(toKey);
      return new StandardRowSortedTable<R, C, V>(sortedBackingMap().headMap(toKey), factory)
          .rowMap();
    }

    @Override
    public SortedMap<R, Map<C, V>> subMap(R fromKey, R toKey) {
      checkNotNull(fromKey);
      checkNotNull(toKey);

      standardClear();
    }

    @Override
    public boolean isEmpty() {
      return standardIsEmpty();
    }

    @Override
    public SortedMap<K, V> subMap(K fromKey, K toKey) {
      return standardSubMap(fromKey, toKey);
    }

    @Override
    public @Nullable Entry<K, V> lowerEntry(K key) {
      return standardLowerEntry(key);
    }

    @Override
    public @Nullable K lowerKey(K key) {

  @Override
  public SortedMap<K, V> subMap(@ParametricNullness K fromKey, @ParametricNullness K toKey) {
    return subMap(fromKey, true, toKey, false);
  }

  @Override
  public SortedMap<K, V> headMap(@ParametricNullness K toKey) {
    return headMap(toKey, false);
  }

  @Override
  public SortedMap<K, V> tailMap(@ParametricNullness K fromKey) {
    return tailMap(fromKey, true);

    return headMap(toKey, false);
  }

  /**
   * This method returns a {@code ImmutableSortedMap}, consisting of the entries whose keys are less
   * than (or equal to, if {@code inclusive}) {@code toKey}.
   *
   * <p>The {@link SortedMap#headMap} documentation states that a submap of a submap throws an
   * {@link IllegalArgumentException} if passed a {@code toKey} greater than an earlier {@code

      }
    }

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      synchronized (mutex) {
        return navigableMap(delegate().headMap(toKey, inclusive), mutex);
      }
    }

    @Override
    public SortedMap<K, V> headMap(K toKey) {
      return headMap(toKey, false);
    }

    @Override

      }
    }

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      synchronized (mutex) {
        return navigableMap(delegate().headMap(toKey, inclusive), mutex);
      }
    }

    @Override
    public SortedMap<K, V> headMap(K toKey) {
      return headMap(toKey, false);
    }

    @Override