Well, a tautology can be made up of non-tautological things; we could conceivably have some sentence phi(p, q) that’s a tautology if p ⇔ q, such that P(p) = f(P(phi(p,q))) = f(P(phi(q,p))) = P(p). I think this is what ygert is trying to do. I don’t have much hope for this approach, though.
Well, a tautology can be made up of non-tautological things; we could conceivably have some sentence phi(p, q) that’s a tautology if p ⇔ q, such that P(p) = f(P(phi(p,q))) = f(P(phi(q,p))) = P(p). I think this is what ygert is trying to do. I don’t have much hope for this approach, though.