Discussion on answer by user21820: How do chess engines decide which best line to play when the game outcome is within their horizon?

@Annatar: I've added a paragraph to my answer touching briefly on the manner in which computing E1 can be expensive, as well as why it is not relevant to the question about known game outcomes. However, it is not true that we need to 'predict which error the opponent makes'. The normal search function is something like Eval(P,α,β,d) where P is a position and α,β are the alpha-beta bounds and d is the depth. To compute E1 we just need to add boolean parameters for whether the players have made an error or not.