A filter accepts and returns a stanza, but potentially modified.
To prevent sending/receiving a stanza, a filter may return None.
Incoming:
self.add_filter('in', in_filter)
Outgoing:
self.add_filter('out', out_filter)
Filters are applied in the order thay are added. However, you may
add an order parameter, which is the place in the list to insert the
filter:
self.add_filter('in', in_filter, order=0)
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