I have posted this in another form on SO 
(http://stackoverflow.com/questions/42565640/how-to-distinguish-between-structural-changes-to-array-made-in-parent-element-vs)
 
and in the 2.0-preview branch slack channel, but I know they are heads down 
with the upcoming release, so hopefully somewhere in the broader group here 
will have insight to share.

The TL;DR question: how many times should the observer, defined in 
editor-element.html log to console for the following sample code: 
https://plnkr.co/edit/iTZqM4GwpASEqQgtRGEk?  I would have expected 4, but 
it actually happens 6 times.

More details: the elements are a list element which, using dom-repeat, 
renders a list of todos, and each todo is rendered by an instance of an 
editor element.  The data is a list of todo objects, each of which has a 
primitive property called description.  First, two todo objects are pushed 
onto the list.  Then the first todo object's description property is 
edited.  Then a third todo object is unshifted (not pushed) onto the list. 
 The data binding is defined two way between the items inside the 
dom-repeat of the list element and the todo property of the editor element. 
 In other words, the binding is made at the object, not the property.  The 
editor element defines an observer on "todo.description", printing a 
message to the console each time an observation is made.

thanks in advance for any help

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