'(internal) Returns an sexp: (unpersistable "reason").  Utility method
for making note that a particular object could not be serialized.'

def unpersistable(self, reason, sxp=None):

    if (sxp is None):
        sxp = []
    sxp.append(unpersistable_atom)
    if isinstance(reason, unicode):
        reason = reason.encode('utf-8')
    sxp.append(reason)
    return sxp
