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

Concurrent Euclid

    A concurrent extension of a subset of
   Euclid ("Simple Euclid") developed by J.R. Cordy and
   R.C. Holt of the University of Toronto in 1980.

   Concurrent Euclid features separate compilation, modules,
   processes and monitors, signal and wait on condition
   variables, 'converters' to defeat strong type checking,
   absolute addresses.  All procedures and functions are
   re-entrant.  TUNIS (a Unix-like operating system) is
   written in Concurrent Euclid.

   ["Specification of Concurrent Euclid", J.R. Cordy & R.C. Holt,
   Reports CSRI-115 & CSRI-133, CSRI, U Toronto, Jul 1980,
   rev. Aug 1981].

   ["Concurrent Euclid, The Unix System, and Tunis," R.C. Holt,
   A-W, 1983].

   (2005-02-19)