docs: function EventExtender (meta, evtExtFn) {
    Definition.call(this, meta);
  
    meta = meta || {};
  
    if (!evtExtFn || !(_.isFunction(evtExtFn))) {
      var err = new Error('extender function not injected!');
      debug(err);
      throw err;
    }
  
    this.version = meta.version || 0;
    this.aggregate = meta.aggregate || null;
    this.context = meta.context || null;
    this.payload = meta.payload || null;
    this.id = meta.id || null;
  
    this.evtExtFn = evtExtFn;
  }