How to control activities on the "back button queue"
<p>I want to be able to control which activities the user can press the back button into, depending on where the user is currently.  For example, I have Activities A, B, C, and D.</p>

<p>The user navigates (through buttons I provide) from Activity A, to B, to C, and finally, to D.  If the user presses back any time before D, I want the normal back operation (if they press back on Activity C, they will be presented with B).</p>

<p>However, if they make it all the way to D, I want to finish activities B and C.  Now, when the user clicks back, I want them to be presented with A.</p>
