beta-eta-contract, to respect "first_order_match"'s specification;
Sledgehammer's Skolem cache sometimes failed without the contraction
By chapter:
chapters 1 and 2, maybe 3: Peter White
chapters 6, 7 and 8: David von Oheim
chapter 6, maybe 8: Gerwin Klein
chapters 3, maybe 4: Tanja Vos
chapters 1-3 at least: Stefano Bistarelli