\documentstyle[fuzz]{article}
\begin{document}
Let $PERSON$ be the set of all people:
\begin{zed}
	[PERSON].
\end{zed}
A `club' has a set of members and a president, who must be a member:
\begin{schema}{Club}
	members: \power PERSON \\
	president: PERSON
\where
	president \subseteq members
\end{schema}
To enroll somebody in the club, we just add them to the set of
members:
\begin{schema}{Enroll}
	\Delta Club \\
	new?: PERSON
\where
	members' = members \cup new? \\
	president' = president
\end{schema}
The president doesn't change when a new member is enrolled.
\end{document}