code: static int hashCodeImpl(Set<?> s) {
      int hashCode = 0;
      for (Object o : s) {
        hashCode += o != null ? o.hashCode() : 0;
  
        hashCode = ~~hashCode;
        // Needed to deal with unusual integer overflow in GWT.
      }
      return hashCode;
    }
docs: An implementation for {@link Set#hashCode()}.