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11 The Worklet Service 323
set, because the validity of a child rule node depends on the conditional expressions
of its ancestors. 2
When a tree is added to the tree set:
If it is a case-level tree, the rule type that the tree represents will be removed from
the Rule Type list. That is, the rule type now has a tree defined for it and so is no
longer available for selection on the New Rule form.
If it is an item-level tree, the task name that the tree represents will be removed
from the Task Name list. That is, the task now has a rule tree defined for it (for
the selected rule type) and so is no longer available.
If it is an item-level tree, and all tasks of the specification now have trees defined
for them for the selected rule type (i.e., this was the final task of the specification
for which a tree was defined), the rule type that the tree represents will be
removed from the Rule Type list.
This approach ensures that rule trees can only be added where there are currently
no trees defined for the selected specification. Once the tree is added, the form resets
to allow the addition of another new tree as required, by repeating the process above
for a new rule type (or rule type/task name for item-level trees).

11.10.4 Drawing a New Exlet

As described in Sect. 11.10.1, adding a conclusion to a selection rule is a matter of
choosing a worklet from the list or creating a new worklet in the Editor. However,
when adding a conclusion for a rule type other than “Selection” (i.e., an exception
rule type), an exlet needs to be defined that will be invoked when the rule is returned
as the last satisfied. Chapter 5 detailed the various actions that make up the available
set of exception handling primitives (or tasks) that may be sequenced to form an
entire handling process. The Draw Conclusion dialog makes the process of defining
an exception handling sequence easier by allowing administrators to create an exlet
graphically by selecting the appropriate primitive from the toolbox on the left, and
then clicking on the drawing canvas to place the selected primitive. Figure 11.11
shows an example of the Draw Conclusion dialog.
In addition to the various primitive tools, the Arc Tool is used to define the
sequence order. For a conclusion to be valid (and thus permitted to be saved), there
must be a direct, unbroken path from the start node to the end node (the start and
end nodes are always displayed on the drawing canvas). Also, the conclusion will
be considered invalid if there are any nodes on the canvas that are not attached to
the sequence when a save is attempted.
The Select Tool is used to move placed primitives around the canvas. The Align
button will immediately align the nodes horizontally and equidistantly between the

2
There are algorithms defined in the literature for the reduction and transformation of Ripple-
Down Rule sets that may be applied from time to time (e.g., see [234]).