'Get the default (unqualified namespace).
This is the expns of the first node (looking up the tree)
that has it set.
@return: The namespace of a node when not qualified.
@rtype: (I{prefix}, I{name})'

def defaultNamespace(self):

    p = self
    while (p is not None):
        if (p.expns is not None):
            return (None, p.expns)
        else:
            p = p.parent
    return Namespace.default
