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Interaction operators
appear inside the pentagon. Interaction operands are separated by dashed lines.
Depending on the operator, there are interaction conditions as well. They appear
at the beginning of the interaction fragment near the fi rst lifeline ( Figure 3.74 ).
In total, there are 12 different interaction operators:
?–  Branches and loops: alt , opt , break , loop .
?–  Concurrency and order: seq , strict , par operator .
?–  Filters and constraints: critical , neg , assert , consider , ignore .
I will explain only a few selected operators in more detail below. You fi nd a
full description in [61].
Alternative fl ows are described with the interaction operator alt . There are
two operands modeled in Figure 3.74 . If subsystem 1 was initialized successfully,
then subsystem 2 is also initialized. Otherwise, subsystem 1 is restarted.
The interaction operator opt is comparable to the operator alt . It is subject to
the constraint that only one operand is permitted, which means that this is the
operand that describes an optional fl ow.


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