reverted e7a80c6752c9 -- there's not much point in putting a diagnosis tool (as opposed to a proof method) in Plain, but more importantly Sledgehammer must be in Main to use SMT solvers
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