Action-expression: action to be performed, e.g., operation calls
that increment attribute values. Events may take arguments
(entry, exit, do take no arguments).
Triggerless transitions have no event between two states in the state
diagram; state change occurring when the internal actions in a state
is completed. When all the actions in a state are performed a
transition without an external event will automatically be
triggered. Action-expressions follow the ``/'' in state diagrams.