Minor edits

Advanced/Definable Sets/parts/4 equivalence.tex

@@ -7,7 +7,7 @@ This is read \say{$S$ satisfies $\varphi$}
 
 \definition{}
 Let $S$ and $T$ be structures. \par
-We say $S$ and $T$ are \textit{equivalent} and write $S \equiv T$ if for any formula $\varphi$, $S \models \varphi \Longleftrightarrow T \models \varphi$. \par
+We say $S$ and $T$ are \textit{equivalent} (and write $S \equiv T$) if for any formula $\varphi$, $S \models \varphi \Longleftrightarrow T \models \varphi$. \par
 If $S$ and $T$ are not equivalent, we write $S \not\equiv T$.
 
 \problem{}