1972: The first Prolog interpreter
at the University of Marseille

Basically, Prolog is theorem prover à la Robinson, restricted to the Horn clause subset of first-order logic. Its programs are made up of implication rules:

conclusion₀    if    condition₁  and  condition₂  and ...   condition_n.

Procedural interpretation:

to prove conclusion₀,
prove condition₁  then  prove condition₂  then ...  prove condition_n.

To demonstrate a goal, the engine follows a linear proof strategy, with backtracking.

Description of the preliminary Prolog:

• Un système de communication homme-machine en Français, by A. Colmerauer et al.,
  Groupe de recherche en Intelligence Artificielle, Faculté des Sciences de Luminy, 1973.

Manual of the final version:

• Prolog, manuel de référence et d'utilisation, by Ph. Roussel,
  Groupe de recherche en Intelligence Artificielle, Faculté des Sciences de Luminy, 1975.

"Most Prolog systems in the world owe their parentage to the original Marseille interpreter" (W.F. Clocksin & C.S. Mellish).

---