Definition:Occurrence (Predicate Logic)

Let $$\mathbf{A}$$ be a WFF of predicate calculus.

Let $$S$$ be a string in the alphabet of predicate calculus.

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.