Definition:Universal Closure of Well-Formed Formula

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Let $\LL_1$ be the language of predicate logic.

Let $\mathbf A$ be a well-formed formula of $\LL_1$.

A universal closure of $\mathbf A$ is a sentence $\mathbf B$ of $\LL_1$ of the form:

$\forall x_1: \cdots \forall x_n: \mathbf A$

By definition of sentence, this means that at least the variables occurring freely in $\mathbf A$ should be quantified over.