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The only purpose of a merge
is to unite optional fl ows to one single strand. It neither checks nor synchronizes
anything ( Figure 2.58 ).
A closer look at each of the use case steps reveals that one single step can, in
turn, contain a complex fl ow. You can also use an activity that is invoked in that
step to model this fl ow. To make sure this is visible in the diagram, we denote the
calling action with a small fork in the lower right corner. This symbol represents a
call hierarchy ( Figure 2.58 ).
Many things happen in our second use case step, check usage right . A message
is sent to the reservation center, a response is expected and received, and sometimes
emergency driving (see Project dairy with Project time 6269 on p. 75) is
triggered. We detail this step in a new activity of type ?«essential?» ( Figure 2.58 ).
First, we use the light-emitting diodes of the status display unit to show the
customer that the system is on and that they should be patient. To this end, we
send a corresponding signal to the status display unit.


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