guava-gwt/src-super/com/google/common/collect/super/com/google/common/collect/ImmutableSet.java
static <E> ImmutableSet<E> unsafeDelegate(Set<E> delegate) {
    switch (delegate.size()) {
      case 0:
        return of();
      case 1:
        return new SingletonImmutableSet<E>(delegate.iterator().next());
      default:
        return new RegularImmutableSet<E>(delegate);
    }
  }
static <E> ImmutableSet<E> unsafeDelegate(Set<E> delegate) {
    switch (delegate.size()) {
      case 0:
        return of();
      case 1:
        return new SingletonImmutableSet<E>(delegate.iterator().next());
      default:
        return new RegularImmutableSet<E>(delegate);
    }
  }
the elements are known to be non-null.