'Convert the nodes nsprefixes into a set.
@param n: A node.
@type n: L{Element}
@return: A set of namespaces.
@rtype: set'

def pset(self, n):

    s = set()
    for ns in n.nsprefixes.items():
        if self.permit(ns):
            s.add(ns[1])
    return s
