The set of bool_interfaces provides default behaviour for
checking/setting the presence of empty subelements.
The prime example of this would be:
bool_interfaces = set(['required'])
This would mean that ``stanza['required']`` would return ``True`` for:
<stanza>
<required />
</stanza>
and ``False`` for:
<stanza />
Likewise, assigning ``stanza['required'] = True`` would add an empty
``<required />`` element, and setting it to ``False`` would remove
such an element if it exists.
a1ddd88208