### 4.2 The procedure box model

The procedure box model of Prolog execution provides a simple way to show the control flow. This model is very popular and has been adopted in many Prolog systems (e.g. SICStus Prolog, Quintus Prolog,…). A good introduction is the chapter 8 of “Programming in Prolog” of Clocksin & Mellish [2]. The debugger executes a program step by step tracing an invocation to a predicate (`call`) and the return from this predicate due to either a success (`exit`) or a failure (`fail`). When a failure occurs the execution backtracks to the last predicate with an alternative clause. The predicate is then re-invoked (`redo`). Another source of change of the control flow is due to exceptions. When an exception is raised from a predicate (`exception`) by `throw/1` (section 6.2.4) the control is given back to the most recent predicate that has defined a handler to recover this exception using `catch/3` (section 6.2.4). The procedure box model shows these different changes in the control flow, as illustrated here:

Each arrow corresponds to a port. An arrow to the box indicates that the control is given to this predicate while an arrow from the box indicates that the control is given back from the procedure. This model visualizes the control flow through these five ports and the connections between the boxes associated with subgoals. Finally, it should be clear that a box is associated with one invocation of a given predicate. In particular, a recursive predicate will give raise to a box for each invocation of the predicate with different entries/exits in the control flow. Since this might get confusing for the user, the debugger associates with each box a unique identifier (i.e. the invocation number).

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