code: def register_dependency(self, data_src, data_sink):
          
  
          pdebug("registering dependency %s -> %s" % (data_src, data_sink))
  
          if (data_src not in self._gettask(data_sink).depends_on):
              self._gettask(data_sink).depends_on.append(data_src)
  
          if (data_sink not in self._gettask(data_src).provides_for):
              self._gettask(data_src).provides_for.append(data_sink)
docs: registers a dependency of data_src -> data_sink
              by placing appropriate entries in provides_for and depends_on