The Free On-line Dictionary of Computing (30 December 2018):

Extensible Shell

    (es) A Unix shell written by Byron
   Rakitzis  and Paul Haahr ,
   derived from rc.  Es has real functions, closures,
   exceptions and lets you redefine most internal shell
   operations.

   Version: 0.84.

   (ftp://ftp.sys.utoronto.ca/pub/es/).

   ["Es - A Shell with Higher Order Functions", P. Haahr et al,
   Proc Winter 1993 Usenix Technical Conference].

   (1993-04-30)