Extend Theory to Satisfy Witness Property/Lemma

Lemma
Let $\LL$ be a language of predicate logic.

Let $\FF$ be a set of $\LL$-sentences.

Suppose that, for every $\LL$-WFF $\phi$ of $1$ free variable $x$:
 * There exists some term $t_\phi$ containing no variables such that:
 * $\paren {\exists x : \map \phi x} \implies \map \phi {x := t_\phi}$
 * is an element of $\FF$.

Then $\FF$ satisfies the witness property.

Proof
Let $\phi \in \FF$ be an arbitrary $\LL$-WFF of $1$ free variable $x$.

Suppose that $\FF \models_{\mathrm{PL}} \exists x : \map \phi x$.

Then, every $\LL$-structure $\MM$ satisfies:
 * $\map {\operatorname{val}_\MM} {\exists x : \map \phi x} = \top$

Additionally, by the assumption on $\FF$:
 * $\map {\operatorname{val}_\MM} {\paren {\exists x : \map \phi x} \implies \map \phi {x := t_\phi}} = \top$

By examining the truth function of conditional, we can conclude that:
 * $\map {\operatorname{val}_\MM} {\map \phi {x := t_\phi}} = \top$

As this holds for every $\MM$, we conclude that:
 * $\FF \models_{\mathrm{PL}} \map \phi {x := t_\phi}$

As $\phi$ was arbitrary, the witness property holds.