code: function eq(vm) {
  	var observerData = vm._mobxObserver;
  
  	if (observerData.stale)
  		return false;	// Re-render.
  	else if (observerData.eq)
  		return observerData.eq.apply(this, arguments); // Let diff() choose.
  	else
  		return true;	// By default: no re-render.
  }
docs: The diff.eq() assigned to each observer vm: