code: def list_blocked_work_units(self, work_spec_name, start=0, limit=None):
          
          return self.registry.filter(WORK_UNITS_ + work_spec_name + _BLOCKED,
                                      start=start, limit=limit)
docs: 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.