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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 list_blocked_work_units(self, work_spec_name, start=0, limit=None):
 """Get a dictionary of blocked work units for some work spec. The dictionary is from work unit name to work unit definiton. Work units included in this list are blocked because they were listed as the first work unit in :func:`add_dependent_work_units`, and the work unit(s) they depend on have not completed yet. This function does not tell why work units are blocked, it merely returns the fact that they are. """

        return self.registry.filter(WORK_UNITS_ + work_spec_name + _BLOCKED,
                                    start=start, limit=limit)