# this line specifies the backend class, but 
# we don't need to know that, to us, its just
# a parameter
my $x = Thing->new( backend => 'A' );

# here we are accessing the backend object
# and calling do on it.
# This to me is the interface for creating the 
# backend object (the "backend" method).
$x->backend->do();

##</code><code>##

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

##</code><code>##

$x->do_backend();

##</code><code>##

  +-------------+                         +------------+
  | Composition |<>---------------------->| Compositor |
  +-------------+                         +------------+
  | Traverse()  |                         | Compose()  |
  | Repair()    |                         +------------+
  +------^------+                                |
         |                       +---------------+-----+
         |                       |                     |
_________|________________   +------------------+  +---------------+
|  compositor->Compose() \|  | SimpleCompositor |  | TeXCompositor |
+-------------------------+  +------------------+  +---------------+
                             | Compose()        |  | Compose()     |
                             +------------------+  +---------------+