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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def num_available(self, work_spec_name):
        '''Get the number of available work units for some work spec.

        These are work units that could be returned by :meth:`get_work`:
        they are not complete, not currently executing, and not blocked
        on some other work unit.

        '''
        return self.registry.len(WORK_UNITS_ + work_spec_name,
                                 priority_max=time.time())