In basic operation, each of the handlers Filter1, Filter2, and Filter3
will make a call to $r->filter_input(), which will return a filehandle.
For Filter1, the filehandle points to the requested file. For Filter2,
the filehandle contains whatever Filter1 wrote to STDOUT. For Filter3,
it contains whatever Filter3 wrote to STDOUT. The output of Filter3
goes directly to the browser.

Note that the modules Filter1, Filter2, and Filter3 are listed in
forward order, in contrast to the reverse-order listing of
Apache::OutputChain.

When you've got this module, you can use the same handler both as a
stand-alone handler, and as an element in a chain. Just make sure that
whenever you're chaining, all the handlers in the chain are "Filter-
aware," i.e. they each call $r->filter_register() exactly once, before
they start printing to STDOUT. There should be almost no overhead for
doing this when there's only one element in the chain.