cant set _properties in setter.js

resourceful=require('./');
X=resourceful.define({
  name:'XXX',
  properties:{
    z:{
      type:'number',
      get:function(val){
        console.log('get','this',this,'val',val);
        return this._properties.z;
      },
      set:function(val){
        console.error('set','this',this,'val',val);
        this._properties.z = 2;
      }
    }
  }
});
a=new X({z:1})
console.error(a)
a._properties.z = 1;
console.error(a)