A secret project by the government (let’s call it Project X) will be executed by one person and consists of 6 tasks: A, B, C, D, E, and F. Figure 2.28 specifies the order in which the tasks need to be executed (precedence graph, cf. PERT/CPM). A possible execution trace is for example ABDCEF.
(a) Model the project in terms of a classical Petri net.
(b) How does one model so that E is optional?
(c) How does one model so that D and E should be executed consecutively, that is, B and C are not allowed between D and E?