the diagrammatic representation in
figure~\ref{fig-iec-diagrams} of a software element in the formalism
of IEC 1131-3 \cite{iec:plc,iee-lew-95}, the industry-standard
programming language for embedded control systems. Software diagrams
in IEC 1131-3 derive directly from circuit notations and, as such,
they encourage thinking in terms of a ``hardware'' paradigm which is
familiar to control engineers. In actuality, however, they
are internally translated into sequential programs and executed by
microprocessors.