as not created in a normal way from the interpreter (x is 
not a command object). To make it function normally, just
evaluate the variable like this:
</p>

<div class="code">
<pre>
% Bar b
% set x [b cget -f]
% $x cget -a
0
%
</pre>
</div>

<p>
In this example, <tt>x</tt> points inside the original structure.  This means that modifications
work just like you would expect.  For example:
</p>

<div class="code">
<pre>

% Bar b
% set x [b cget -f]
% $x configure -a 3            # Modifies contents of f (inside