code: def add(self, observableElement):
          
          if observableElement not in self._observables:
              self._observables.append(observableElement)
          else:
              raise RuntimeError(
                  "{0} is already an observable element"
                  .format(observableElement))
docs: add an observable element
  
          :param str observableElement: the name of the observable element
          :raises RuntimeError: if element name already exist in the store