public final boolean contains(@Nullable Object element) {
    if (this == EMPTY) return false;
    E single = asElement();
    if (single != null) return Comparing.equal(single, element);
    Collection<E> col = asCollection();
    if (col != null) return col.contains(element);
    for (E e : this) {
      if (Comparing.equal(e, element)) return true;
    }
    return false;
  }
public final boolean contains(@Nullable Object element)