Topic: To create unique identifiers for pair elements
There are elements, each of which has a unique identifier (in the elementary case - their address in storage). It is required to create the identifier for pair elements but so that its value did not depend on the order in which they undertake. It would be possible to use XOR, but there is a probability that its value coincides with other pair. And if to blind identifiers instead of by means of OR the result will depend on the order.