code: def get_organisational_tags_to_task(self):
          
          if self._organisational_tags_to_task != {}:
              return self._organisational_tags_to_task
  
          for taskid, task in self.get_tasks().items():
              for tag in task.get_tags()[2]:
                  self._organisational_tags_to_task.setdefault(tag.get_name(), []).append(taskid)
  
          return self._organisational_tags_to_task
docs: This build a dict for fast retrive tasks id based on organisational tags. The form of the dict is:
          
              { 'org_tag_1': ['task_id', 'task_id', ...], 
                'org_tag_2' : ['task_id', 'task_id', ...], 
                ... }