@ElementTypesAreNonnullByDefault
public abstract class ImmutableSet<E> extends ImmutableCollection<E> implements Set<E> {

  /**
   * Returns a {@code Collector} that accumulates the input elements into a new {@code
   * ImmutableSet}. Elements appear in the resulting set in the encounter order of the stream; if
   * the stream contains duplicates (according to {@link Object#equals(Object)}), only the first