Recursive path ordering (RPO) is a well-known reduction ordering introduced by Dershowitz [14], that is useful for proving termination of term rewriting systems (TRSs). Jouannaud and Rubio generalized this ordering to the higher-order case thus creating the higher-order recursive path ordering (HORPO) [19]. They proved that this ordering can be used for proving termination of higher-order TRSs which essentially comes down to proving well-foundedness of the union of HORPO and the β-reduction relation of the simply typed lambda calculus (λ → ). This result entails well-foundedness of RPO and termination of λ → . This paper describes author's undertaking of providing a complete, axiom-free, fully constructive formalization of those results in the theorem prover Coq. Formalization is complete and hence it contains all the dependant results. It can be divided into three parts:• finite multisets and two variants of multiset extensions of a relation,• λ → with termination of β as the main result,• HORPO with proof of well-foundedness of its union with β-reduction. Also decidability of HORPO has been proven and due to the constructive nature of this proof a certified algorithm to verify whether two terms can be oriented with HORPO can be extracted from this proof.CONTENTS