\begin{definition}[Definition:Definable Truth Function]
Let $f: \Bbb B^n \to \Bbb B$ be a truth function.
Let $S$ be a set of truth functions.
Then $f$ is '''definable from $S$''' {{iff}} there exist:
:a truth function $g: \Bbb B^m \to \Bbb B$, obtained by composition of truth functions from $S$
:an injection $i: \Bbb B^n \to \Bbb B^m$
such that:
:$f = g \circ i$
{{expand|example}}
\end{definition}