The sensitivity list of the wait statement specifies a set of signals to
which the process is sensitive while it is suspended. When an event occurs on any of these signals (that is, the value of the signal changes), the process
resumes and evaluates the condition. If it is true or if the condition is
omitted, execution procedes with the next statement, otherwise the process
resuspends. If the sensitivity clause is omitted, then the process is
sensitive to all of the signals mentioned in the condition expression. The
timeout expression must evaluate to a positive duration, and indicates the
maximum time for which the process will wait. If it is omitted, the process
may wait indefinitely.