he state space. The number of state symbols in an EFSM diagram based on explicit states depends on the number of values that the selected variable has. FSM diagram: EFSM diagram: ({undressed) {undressed} post (tie blue) (grey.blue) (white,yellow) {jacket grey}) pre: (tie blue) (grey.yellow pre: {tie yellow} (red,blue) post: {tie blue} pre: {tie yellow} (white,blue) pre: (tie yellow} The explicit state (red,yellow) pre: {tie blue) (jacket red} post: {tie yellow} {jacket white} pre: {tie blue) pre: {tie yellow} {jacket red} post: {tie blue} pre: {tie yellow} Fig. 7.45 Another EFSM representation of the same FSM diagram Another common technique for creating EFSM diagrams is the use of state classes, often referred to as "hierarchical states" in other methods, see Fig. 7.46. Service Resource model: Husband "State::=dressState{Dressed,Undressed} Dressed::=dressed SET {Jacket, Tie} Jacket::=jacket{grey,red,white} Tie::=tie(blue,yellow} Undressed::=undressed {undressed}} The state space Modified service behaviour {undressed} {undressed} {undressed} {undressed} {dressed} {grey,blue} {grey,blue} {white,yellow} {white,yellow} {red,blue} {red,blue} {grey,yellow] {grey,yellow {white,blue} {dressed} {white,blue) {red,yellow} {{red,yellow} Fig. 7.46 Redefining the state space by introducing the state class dressState 7 Concepts and Notations for Process Control Systems Modelling The explicit state technique is based on defining sets of states, where at least one state variable has a constant value. In the state class technique, any set of states can be regarded as a state. This technique requires, however, that such sets of states must be given unique names that are assigned as values to a new state variable. Such a variable is a state class. Definition of state classes must be part of the spec- ification of the state space. In the specification of the state space, a state class is a variable that consists of other variables. For example, in our dressing case, we introduce two subsets of states and give them the name undressed