code: def referenced_by(self, refobj):
          
          try:
              ref = cmds.referenceQuery(refobj, referenceNode=True)
              return ref
          except RuntimeError as e:
              if str(e).endswith("' is not from a referenced file.\n"):
                  return None
              else:
                  raise e
docs: Return the reference that holds the given reftrack node.
  
          Returns None if it is imported/in the current scene.
  
          :param refobj: the reftrack node to query
          :type refobj: str
          :returns: the reference node that holds the given refobj
          :rtype: str | None
          :raises: None