The UK prime minister hopes to replace the Irish backstop with his own "two borders, four years" proposal. There are two elements to his plan. First, he wants Northern Ireland to leave the EU's custom union, which is the bloc's tariff-free trading area. But at the same time he wants Northern Ireland to remain aligned to the EU's single market access rules, which is the bloc's uniform set of regulations for all goods. So how would this work on the ground?