\documentclass{article}

\usepackage[margin=30mm,a4paper]{geometry}
\usepackage{circus}

\newcommand{\opt}{~\textrm{opt}~}
\newcommand{\real}{\mathbb{R}}
\begin{document}

\begin{zsection}
  \SECTION basic\_toolkit \parents circus\_toolkit
\end{zsection}
%
\begin{zed}
[NAME]
\end{zed}
%
%%Zpreword \opt opt
%%Zpreword \real real
%
\begin{axdef}
\real:\power \arithmos\\
\end{axdef}
%
\begin{axdef}
r: \num \cross \nat \fun \real\\
\end{axdef}
%
\begin{zed}
\generic (\opt \varg)
\end{zed}
%
\begin{zed}
\opt X == \{s: \seq X | \# s \leq 1\}
\end{zed}
%
\begin{axdef}
b2r: \boolean \fun \real
\where
\forall v: \boolean @ b2r(v) = (\IF~v = \true \THEN~1\ELSE~0)
\end{axdef}
%
\begin{axdef}
truthvalue: \arithmos \fun \boolean
\where
\forall x: \arithmos | x > 0 @ truthvalue(x) = \true\\
\forall x: \arithmos | x \leq 0 @ truthvalue(x) = \false
\end{axdef}

\end{document}
