Unification Modulo Nonnested Recursion Schemes via Anchored Semi-Unification

Authors Gert Smolka, Tobias Tebbi

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Gert Smolka
Tobias Tebbi

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Gert Smolka and Tobias Tebbi. Unification Modulo Nonnested Recursion Schemes via Anchored Semi-Unification. In 24th International Conference on Rewriting Techniques and Applications (RTA 2013). Leibniz International Proceedings in Informatics (LIPIcs), Volume 21, pp. 271-286, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2013)


A recursion scheme is an orthogonal rewriting system with rules of the form f(x1,...,xn) -> s. We consider terms to be equivalent if they rewrite to the same redex-free possibly infinite term after infinitary rewriting. For the restriction to the nonnested case, where nested redexes are forbidden, we prove the existence of principal unifiers modulo scheme equivalence. We give an algorithm computing principal unifiers by reducing the problem to a novel fragment of semi-unification we call anchored semi-unification. For anchored semi-unification, we develop a decision algorithm that returns a principal semi-unifier in the positive case.
  • recursion schemes
  • semi-unification
  • infinitary rewriting


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