code: function off(event, selector, handler) {
      const enterCallbacks = Object.keys(this.trackedElements[selector].enter || {});
      const leaveCallbacks = Object.keys(this.trackedElements[selector].leave || {});
  
      if ({}.hasOwnProperty.call(this.trackedElements, selector)) {
          if (handler) {
              if (this.trackedElements[selector][event]) {
                  const callbackName = (typeof handler === 'function') ? handler.name : handler;
                  delete this.trackedElements[selector][event][callbackName];
              }
          } else {
              delete this.trackedElements[selector][event];
          }
      }
  
      if (!enterCallbacks.length && !leaveCallbacks.length) {
          delete this.trackedElements[selector];
      }
  }
docs: Stops tracking elements matching a CSS selector. If a selector has no
  callbacks it gets removed.
  
  @param {string} event The event you want to stop tracking (enter or leave)
  @param {string} selector The CSS selector you want to stop tracking
  @return {void}