Xs is a cleanly-designed shell with functional programming. It is based off the source-code for 
the es project, which was in the public domain. Currently, the changes in xs can also be 
considered to be in the public domain, although this will probably change soon. 

Xs is not source-level compatible with es. See the manpage (xs.1) for more details about it.

Most of the xs source code remains untouched from es. The primary authors of that shell can be 
found in that source code, currently located at: ftp://ftp.sys.utoronto.ca/pub/es/. Modifications 
since es-0.9-beta1 are all parts of xs and have been written by Frederic Koehler.

