What is a guard condition? Explain it with a suitable example.

The guard-condition is a Boolean expression written with respect to parameters of triggering event and attributes and links of object which owns state machine. The guard condition can also involve tests of simultaneous states of the current machine (or explicitly designated states of some available object); for case, "in State1" or "not in State2". State names can be fully qualified by nested states which contain them, yielding path names of form ''State1: State2::State3''; this can be used in a case where same state name occurs in various composite state regions of an overall machine.