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.