\begin{definition}[Definition:Occurrence (Predicate Logic)]
Let $\mathbf A$ be a WFF of predicate logic.
Let $S$ be a string in the alphabet of predicate logic.
Each place where $S$ appears in $\mathbf A$ is called an '''occurrence of $S$ in $\mathbf A$'''.
Note that $S$ may consist of a single symbol, but may not be null.
\end{definition}