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<p>Provides implementations of callbacks, objects that encapsulate a server request that is deferred,
typically to allow a user to login or otherwise authenticate before proceeding with
some other activity.

<p>In practice, an implementation of {@link org.apache.tapestry.IPage#validate(IRequestCycle)} or
{@link org.apache.tapestry.IActionListener} will create a callback, and assign it as a
persistent page property of an application-specific login page.  After the login completes, it 
can use the callback to return the user to the functionality that was deferred.

<p>Another example use would be to collect billing and shipping information as part of
an e-commerce site's checkout wizard.

@author Howard Lewis Ship <a href="mailto:hlship@apache.org">hlship@apache.org</a>

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