16. Well-formed formulas (wffs) are defined recursively as follows: T is a wff. F is a wff. Any proposition variable is a wff. If X is a wff, then (-X) is also a wff. If X and Y are wffs, then (X AY) is also a wff. If X and Y are wffs, then (XVY) is also a wff. We say that a formula is in De Morgan normal form if it satisfies the following conditions. ("De Morgan normal form" is not standard terminology; I just made it up.) Every negation in the formula is applied to a variable, not to a more complicated subformula. Either the entire formula is T, or the formula does not contain T. Either the entire formula is F, or the formula does not contain F. Prove that for every wff, there is a logically equivalent wff in De Morgan normal form. For example, the well-formed formula (-((p19) Var))^(-(p V-r)19) is logically equivalent to the following wff in De Morgan normal form: ((p V-) Ar)) ((par)q)