'Generate a I{reverse} mapping of unique prefixes for all namespaces.
@return: A referse dict of prefixes.
@rtype: {u, p}'

def genPrefixes(self):

    prefixes = {}
    n = 0
    for u in self.namespaces:
        p = ('ns%d' % n)
        prefixes[u] = p
        n += 1
    return prefixes
