Guard and Mode-guard

A rule is guarded against triggers by an optional guard G. A guard makes a rule trigger conditional to fulfilment of certain conditions. These conditions are represented as a boolean expression tree T with non-terminal nodes as boolean operators and terminal nodes as calls to JavaScript functions as shown in Figure 4. In addition, every guard has a collection of fields known as mode-guard ${\cal M}_G$ such that a guard will be evaluated if and only if all the fields comprising mode are known. The evaluation of guard ${\it eval}_g$ is defined in terms of the associated boolean expression tree $T_G$ as:

${\it eval}_g$ $\equiv$ ${\it eval}_{T_G}$ : $T_G$ $\rightarrow$ $\mathbb{B}$

Figure 4: A guard is represented as an expression tree with non-terminal nodes as boolean operators.