\documentclass[fleqn,leqno]{article}
\usepackage{hypertlabook}
\pdftitle{ASCII Text}
\fixverbatim
\begin{popup}
\begin{verbatim*}
SetSum(T) == IF T = {} THEN 0
                       ELSE LET t == CHOOSE x \in T : TRUE
                            IN  t + SetSum(T \ {t})
\end{verbatim*}
\end{popup}
\makeasciipopup