Exploits the `skolem_symbol_introduction` check by introducing a Skolem axiom with a free variable.
The verifier checks that all explicit universal variables (`![X]:`) are present in the Skolem term, but fails to check for free variables.
Since free variables are implicitly universally quantified at the top level in TPTP, `(? [Y]: Y = Z) => sk = Z` becomes `![Z]: ((? [Y]: Y = Z) => sk = Z)`.
Because `? [Y]: Y = Z` is a tautology, this unsoundly derives `![Z]: sk = Z`, collapsing the domain to a single element and allowing `$false` to be derived from `a != b`.
