code: def add_ref(self, name, ref):
          
          if self._name is None:
              self._name = name
          elif name != self._name:
              raise RuntimeError('Contexts can only share between backends of '
                                 'the same type')
          self._refs.append(weakref.ref(ref))
docs: Add a reference for the backend object that gives access
          to the low level context. Used in vispy.app.canvas.backends.
          The given name must match with that of previously added
          references.