code: def _reprJSON(self):
          
          return {'__MzmlPrecursor__': (self.spectrumRef, self.activation,
                                        self.isolationWindow, self.selectedIonList
                                        )
                  }
docs: Returns a JSON serializable represenation of a ``MzmlPrecursor``
          class instance. Use :func:`maspy.core.MzmlPrecursor._fromJSON()` to
          generate a new ``MzmlPrecursor`` instance from the return value.
  
          :returns: a JSON serializable python object