# Definition:Substitution (Formal Systems)/Letter

## Definition

Let $\FF$ be a formal language with alphabet $\AA$.

Let $\mathbf B$ be a well-formed formula of $\FF$.

Let $p$ be a letter of $\FF$.

Let $\mathbf A$ be another well-formed formula.

Then the substitution of $\mathbf A$ for $p$ in $\mathbf B$ is the collation resulting from $\mathbf B$ by replacing all occurrences of $p$ in $\mathbf B$ by $\mathbf A$.

It is denoted as $\map {\mathbf B} {\mathbf A \mathbin {//} p}$.

Note that it is not immediate that $\map {\mathbf B} {\mathbf A \mathbin {//} p}$ is a well-formed formula of $\FF$.

This is either accepted as an axiom or proven as a theorem about the formal language $\FF$.