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