application of the theorem-prover treats such clauses as procedures:

to show/solve H, show/solve B₁ and ... and B_n.

Pure Prolog was soon extended, however, to include negation as failure, in which negative conditions of the form not(B_i) are shown by trying and failing to solve the corresponding positive conditions B_i.

Subsequent extensions of Prolog by the original team introduced constraint logic programming

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