Multi-letter identifiers have been changed to look better than they do with vanilla LATEX: instead of specifications, you get specifications. The letters haven't been spread apart, and the ligature fi has been used. This is in typewriter font
\begin{schema}{BirthdayBook} known: \pset NAME \\ birthday: NAME \pfun DATE \ST known = \dom birthday \end{schema}

\begin{axdef} limit : \nat \ST limit \leq 65536 \end{axdef}

\begin{class}{Shape} \also colour : Colour \\ \end{class}