Ancestral Implementations
-------------------------

Xs is based on the source code for the es project, which was derived
from rc. All of these are in the public domain.

The es shell source code is currently located at:
ftp://ftp.sys.utoronto.ca/pub/es/ .

Other es information is at:
https://wryun.github.io/es-shell/
https://wryun.github.io/es-shell/paper.html
https://wryun.github.io/es-shell/manpage.html

Information regarding rc is at:
http://tobold.org/article/rc
http://static.tobold.org/rc/rc-duff.html
