code: function(fn, context){
        var cursor = this;
        while (cursor = cursor.handler)
          if (cursor.fn === fn && cursor.context === context)
            console.warn('duplicate fn & context pair');
  
        this.handler = {
          fn: fn,
          context: context,
          handler: this.handler
        };
      }
docs: Add callback on token value changes.
  @param {function(value)} fn
  @param {object=} context