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2 The Language: Rationale and Fundamentals 63

Net
(id)
... is top-level net

.../has end-... .../has start-... .../contains ... decomposes into .../...

Condition
... precedes .../...

NetEltType NetElement
(name) (id)
.../is of ... .../removes ...
{task, condition}

... has join .../... Behaviour
... has default .../... Task
(type)
{AND,OR,XOR}
Number … is ranked … in order ... OR-split ... has split .../...
task
MI Task
is MI
task

XOR-split
.../has ... .../has lowerbound ... .../has threshold ... .../has upperbound ... is a number
task

The default outgoing arc for an XOR-split is the Dynamicity Number Number and
one to the NetElement that is ranked lowest (name) Infinity
{static,dynamic} N∪ {∞}
Fig. 2.25 Object role model: YAWL control-flow perspective

There is a precedence relationship that provides an ordering between all net ele-
ments such that every net element is on a path from the start to the end condition.
Net elements are connected using directed arcs that indicate the direction in which
the thread of execution flows. The set of directed arcs for a net is termed the flow
relation.
Where a net element is a task, it may demonstrate split or join behavior of AND,
OR, or XOR type. Also, a task may be atomic in form or it may have multiple
instances. Where it is a multiple instance task, it has a lower and upper bound speci-
fied, which indicate the minimum and maximum number of instances that may run.
It also has a threshold, which indicates how many instances must be complete before
the thread of control can be passed on, and a dynamicity, which indicates whether
the task is static, that is, all instances are started at commencement or dynamic,