code: def parents(self, child):
          
          if child not in self._child_to_parents:
              raise ValueError("Child sensor %r not in tree." % child)
          return self._child_to_parents[child].copy()
docs: Return set of parents of child.
  
          Parameters
          ----------
          child : :class:`katcp.Sensor` object
              Child whose parents to return.
  
          Returns
          -------
          parents : set of :class:`katcp.Sensor` objects
              The parent sensors of child.