A Pragmatic Reconstruction of LambdaProlog

λProlog is a logic programming language in which hereditary Harrop formulas generalise Horn formulas, and simply typed λ-terms generalise Prolog terms. One may wonder whether these extensions are simultaneously required, and if it exists useful subsets of λProlog, at least for pedagogical purposes. We answer this question by exhibiting a network of necessity links between the new features of λProlog. A handy programming heuristic can be proposed after these necessity links for defining relations by induction on types