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) {

    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<K, V> subMap(@ParametricNullness K fromKey, @ParametricNullness K toKey) {
      return asMap(backingSet().subSet(fromKey, toKey), function);
    }

    @Override
    public SortedMap<K, V> headMap(@ParametricNullness K toKey) {
      return asMap(backingSet().headSet(toKey), function);
    }

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

    }

    @Override
    public SortedMap<K, V> subMap(@ParametricNullness K fromKey, @ParametricNullness K toKey) {
      return asMap(backingSet().subSet(fromKey, toKey), function);
    }

    @Override
    public SortedMap<K, V> headMap(@ParametricNullness K toKey) {
      return asMap(backingSet().headSet(toKey), function);
    }

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