The prolog phenomenon

In 1968, Carl Hewitt introduce PLANNER, a "procedural deductive system." [Hewitt 72] It featured some very innovative concepts for the theorem-proving community of that time: the procedural interpretation of deduction, pattern-directed procedure invocation, an indexed data base of assertions and programs, and nondeterminism (backtracking). A simple version was implemented by Sussman, Winograd and Charniak [Sussman 71] and used by Winograd in his exciting work on natural language. [Winograd 72] A very similar language, QA4, was implemented on the West Coast. [Rulifson 72] These languages, known by the ambitious term "AI languages," were widely perceived as providing built-in facilities that everyone would need for the next generation of AI systems, the way LISP had provided facilities for the previous one.