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Let $\mathbf C$ be a WFF of predicate logic.
Let $x$ be a variable in $\mathbf C$.
- $( Q y_i: \mathbf B )$
where $Q$ is a quantifier and $\mathbf B$ is a WFF.
Also known as
One often sees free for as a convenient abbreviation for freely substitutable for.
- Confusion of Bound Variables, showing what goes wrong when substituting a variable that is not free for another.