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# this line specifies the backend class, but 
# we don't need to know that, to us, its just
...
# This to me is the interface for creating the 
# backend object (the "backend" method).
$x->backend->do();

$x->backend->new()->do();

$x->do_backend();

  +-------------+                         +------------+
  | Composition |<>---------------------->| Compositor |
...
+-------------------------+  +------------------+  +---------------+
                             | Compose()        |  | Compose()     |
                             +------------------+  +---------------+