In the attached image, I am having trouble with point 66. I realize the notation is a bit archaic, so I'll explain as much as I can. [(x,y)∈]a means "the set of solutions of the proposition a(x,y)." For example, the set of solutions to the proposition x < y. -= means "is not equivalent to", and refers to two sets in each case here. Λ means "the empty set". The period is part of dot-parentheses notation. I'll tie everything together in parentheses in a little bit rather than explain it. = means "if and only if" and links two propositions. The next part is what bothers me. I think [y∈]a is the set of solutions of a(x,y) in terms of x alone. But, what does this really mean? What would the resultant set be if a were the proposition x<y? What if it were x⇒y? I suspect that this has some ties to lambda calculus, but that doesn't help me much since I have no real knowledge of it. Now, all together, the tautology is: ([(x,y)∈]a -= Λ) = (([x∈]([y∈]a -= Λ)) -= Λ) Can anyone help me understand the partial solution [y∈]a?