return ImmutableMap.this.keySet();
    }

    @Override
    public boolean containsKey(@Nullable Object key) {
      return ImmutableMap.this.containsKey(key);
    }

    @Override
    public @Nullable ImmutableSet<V> get(@Nullable Object key) {
      V outerValue = ImmutableMap.this.get(key);
      return (outerValue == null) ? null : ImmutableSet.of(outerValue);
    }

    @Override

    }

    /**
     * Associates {@code key} with {@code value} in the built map. If the same key is put more than
     * once, {@link #buildOrThrow} will fail, while {@link #buildKeepingLast} will keep the last
     * value put for that key.
     */
    @CanIgnoreReturnValue
    public Builder<K, V> put(K key, V value) {
      ensureCapacity(size + 1);
      checkEntryNotNull(key, value);

    }

    /**
     * Associates {@code key} with {@code value} in the built map. If the same key is put more than
     * once, {@link #buildOrThrow} will fail, while {@link #buildKeepingLast} will keep the last
     * value put for that key.
     */
    @CanIgnoreReturnValue
    public Builder<K, V> put(K key, V value) {
      ensureCapacity(size + 1);
      Entry<K, V> entry = entryOf(key, value);