# Definition:Occurrence (Formal Systems)

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*This page is about Occurrence in the context of formal systems. For other uses, see Occurrence.*

## Definition

Let $\FF$ be a formal language.

Let $S, T$ be collations in the alphabet of $\FF$.

Each place where $S$ appears in $T$ is called an **occurrence of $S$ in $T$**.

## Also known as

Some sources refer to this as an **instance**.

## Sources

- 1964: Donald Kalish and Richard Montague:
*Logic: Techniques of Formal Reasoning*... (previous) ... (next): $\text{II}$: 'AND', 'OR', 'IF AND ONLY IF': $\S 4$ - 1965: E.J. Lemmon:
*Beginning Logic*... (previous) ... (next): Chapter $2$: The Propositional Calculus $2$: $1$ Formation Rules