Asynchronous diagrams, and in particular the C-element, are accurately described using Petri nets (Spars and Furber 2002, p. 86) or process algebras (Wang and Kwiatkowska 2007). We present a (1-bounded) Petri net for the C-element in Fig. 5c, in which transitions are interpreted as signal transitions and places and arcs capture the causal relations between the signal transitions. Following the usual convention, the Petri net is drawn in simpler form where most places have been omitted. We can observe that both tokens are needed in order to excite the transitions that cause the event \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Z_{hi}$$\end{document}Zhi, which in turn will require the events \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$X_{hi}$$\end{document}Xhi and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Y_{hi}$$\end{document}Yhi to be triggered. The same is true for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Z_{lo}$$\end{document}Zlo.