removed fixed_tr: now handled in syntax module;
replaced mixfix_typ by TypeInfer.mixfixT, which handles binders as well;
def: beta/eta contract lhs;
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