什么叫做父Behavior?

//page index
Page({
  behaviors:[PageBehavior]
});

//PageBehavior
Behavior({
  data:{
      tval:'Page'
  },
  behaviors:[BaseBehavior]
});

//BaseBehavior
Behavior({
  data:{
      tval:'Base'
  }
});

请问`PageBehavior`和`BaseBehavior`谁是父Behavior?