 Any changes to <code>parentModel</code> will be reflected
in <code>localModel</code> and any changes in <code>localModel</code> will reflect in <code>parentModel</code>. If the parent
scope property doesn&#39;t exist, it will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception. You
can avoid this behavior using <code>=?</code> or <code>=?attr</code> in order to flag the property as optional. If
you want to shallow watch for changes (i.e. $watchCollection instead of $watch) you can use
<code>=*</code> or <cod