'A sequence containing each of the namespace prefixes appearing in
this tree. Each prefix appears once and only once, and in document
order.'

@property
def nspfxs(self):

    def merge(seq, seq_2):
        for item in seq_2:
            if (item in seq):
                continue
            seq.append(item)
    nspfxs = [self.nspfx]
    for child in self._children:
        merge(nspfxs, child.nspfxs)
    return nspfxs
