guava/src/com/google/common/collect/ImmutableSet.java
public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4) {
    return construct(4, 4, e1, e2, e3, e4);
  }
public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4) {
    return construct(4, 4, e1, e2, e3, e4);
  }
Returns an immutable set containing the given elements, minus duplicates, in the order each was
first specified. That is, if multiple elements are {@linkplain Object#equals equal}, all except
the first are ignored.