\documentclass[fleqn,leqno]{article}
\usepackage{hypertlabook}
\pdftitle{ASCII Text}
\fixverbatim
\begin{popup}
\begin{verbatim*}
RECURSIVE SumSeq(_)       
SumSeq(s) == IF s = << >> THEN 0
                          ELSE Head(s) + SumSeq(Tail(s))
\end{verbatim*}
\end{popup}
\makeasciipopup