code: def __x_product (property_sets):
      
      assert is_iterable_typed(property_sets, property_set.PropertySet)
      x_product_seen = set()
      return __x_product_aux (property_sets, x_product_seen)[0]
docs: Return the cross-product of all elements of property_sets, less any
          that would contain conflicting values for single-valued features.