OPS-5 and variants are widely used expert system tools.

Advantage:

1. The tool does not greatly restrict what one can do.

Disadvantages:

1. Low-level code: the ``assembly language'' of expert system tools.
2. Kinky code: Although OPS-5 supposedly offers flexible control, rules tend to restrict this control by assertion of ``control variables''.

Free OPS-like program in C: CLIPS
http://www.ghg.net/clips/CLIPS.html

(defrule reactor_pressure_demon
  "Reactors blow if pressure too high"
  (object (is-a reactor)
          (name ?n)
          (pressure ?p:(> = ?p 610)))
  => 
  (send ?n blown))

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