Some code derived from xasteroids Copyright Phil Goetz.
See the file README.xast, which is the README file from the xasteroids

All other code Copyright 1994 Brad Pitzel
Feel free to use/distribute/modify as long as acknowledgement to
sasteroids author(s) is made.

From README.xast:

     If you modify the game, feel free to post your version, PROVIDED that
you retain my copyright notice, the credits, and note which version yours
was derived from and its release date, what changes you made, and your
release date.  I do not intend to release any more versions myself.
I wash my hands of it.

Thank you!