The Substitution Rule In Operation

Example

Let be the tautology P v -P, and let the substitution for P be A v -B

Then by the substitution rule

(A v -B) v -(A v -B)

is also a tautology

N.B. For the substitution to work, it has to operate upon a tautology.

How can we hook up a tautology to each of our equivalences?