[bdbxml] Indexes and Booleans

I have a node in my xml document that holds boolean data.  The value for
this node will either by "true" or it will be empty (the node will
exist, but will have no content).  I would like to put an index on this
node.  I have two questions about how I should set up this index:
 
(1) Should this be a KEY_EQUALITY type or a KEY_PRESENCE type?  I didn't
know if the node existed, but was empty, whether that would be
considered "present" or not.  I assume if it's KEY_PRESENCE, that the
syntax (question #2) does not matter so much.
 
(2) Is the value of "true" or empty satisfactory to support a syntax of
XmlValue.BOOLEAN, or should I just make this a STRING?