'True if the namespace has a value, or if it\'s the prefix of one that
does.'

def is_prefix(self, namespace):

    (first, sep, rest) = namespace.partition(self._sep)
    if (not first):
        return True
    elif (first not in self._subspaces):
        return False
    else:
        return self._subspaces[first].is_prefix(rest)
