Exercise 4. In the definition of the nested sequence of A's in the preceding proof, we did not

write:

Ait1

[A₁U {4₁}

A; U{-₁}

if A, U {₁} is finitely satisfiable,

if A, U{-₁} is finitely satisfiable.

Explain why. Hint: Exhibit a set I of wff's and a single wffy such that both IU {p} and

TU{-} are satisfiable.