EXPLAINER: For UK and EU, Brexit has become a messy divorce

LONDON (AP) — When Britons voted for Brexit in 2016, they were promised a smooth transition to a new economic relationship with the European Union. Now the two sides are hurtling toward a tumultuous split that threatens billions worth of trade and hundreds of thousands of jobs. An outcome almost no one wants looks increasingly hard to avoid, with U.K. and EU leaders setting Sunday as the deadline for a “firm decision” about the future of the…