license for public domain software?

[Leo Singer]

Hello,

What should the 'license' be for software that is in the public domain? I am updating py-novas_py; the license says:

This software was produced by the United States Naval Observatory at the expense of United States taxpayers, and is therefore not suseptible to copyright, because a copyright would place taxpayer property under private ownership. Since it is not copyrighted, it cannot be licensed; it is simply free.

Thanks,
Leo Singer
Graduate Student @ LIGO-Caltech