e>parentModel</code> on the parent scope. Changes to <code>parentModel</code> will be reflected in
<code>localModel</code> and vice versa. Optional attributes should be marked as such with a question mark:
<code>=?</code> or <code>=?attr</code>. If the binding expression is non-assignable, or if the attribute isn&#39;t
optional and doesn&#39;t exist, an exception (<a href="error/$compile/nonassign"><code>$compile:nonassign</code></a>)
will be thrown upon discovering changes to the local value, since it will