docs: def remove(self, what, call):
          """
          remove an observer
  
          what: (string | array) state fields to observe
          call: (function) when not given, decorator usage is assumed.
              The call function should have 2 parameters:
              - previousValue,
              - actualValue
  
          """
          type = observerTypeEnum.typeOf(what)
          self._observers.remove({
                                      "observing": what,
                                      "type": type,
                                      "call": call
                                   })