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pay
pay
coin
coin
“tea!“
“coffee!”
“coffee!“
select
select
offer
offer
beverage
beverage
Fig. 7.
Beverage service and a strategy, equipped with ports
coin
“tea!”
“coffee!”
beverage
Fig. 8.
Composition of the port equipped oWFNs of Fig. 7
4.6
Hierarchical Open Workflow Nets
Abstraction and refinement are fundamental construction principles for com-
plex systems: Only a hierarchical design process makes complexity tractable.
oWFN allow for a simple, canonical notion of refinement. The basic idea is the
replacement in
N
of a transition
t
by an other oWFN
M
, written (as usual for
replacement operators)
N
[
M
\
t
]
(“in
N
, replace
M
for
t
”). This is possible whenever the interface of
M
coincides
with the environment
•
t
t
•
of
t
.
As an additional technicality, the addition
m
+
m
of markings
m
and
m
of
oWFN
N
and
N
, respectively, is defined for each
p
∪
∈
P
N
∪
P
N
,by
(
m
+
m
)(
p
)=
m
(
p
)+
m
(
p
)
,
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