import java.util.function.Consumer;
import java.util.stream.Collector;
import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A {@link Set} whose contents will never change, with many other important properties detailed at
 * {@link ImmutableCollection}.
 *
 * @since 2.0
 */
@GwtCompatible(serializable = true, emulated = true)