Traditionally, Retro has been released as public domain. In more recent times, with the increase in contributions from users outside the United States, this has lead to questions as most countries do not recognize the ability of an author to give up copyright protection on his work. Further study implies that even in the USA, outside of works developed by the government, it may not be possible to give up copyright protection.
For this reason, we have released the current generation of Retro under the ISC License, which offers as much freedom as possible. Basically the only requirement is that you leave any copyright statements and license file in the source code.
You are free to use any license you like for your own code, but contributions submitted for inclusion with Retro will be placed under this license. (As an exception, libraries submitted can be under other licenses, but this must be marked in the source files and any included documentation on them so users of Retro can be aware of any potential legal issues with specific libraries)
Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies. THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.