https://www.reuters.com/article/us-apple-epic-games-factbox/factbox-five-things-to-know-about-epics-epic-legal-fight-with-apple-idUSKCN2D228Z >Apple Inc Chief Executive Tim Cook takes the witness stand on Friday to defend the lucrative App Store against "Fortnite" maker Epic Games' allegations that it is a monopoly that Apple illegally abuses.