Encompassment ordering
In theoretical computer science, in particular in automated theorem proving and term rewriting, the containment,[1] or encompassment, preorder (≤) on the set of terms, is defined by[2]
- s≤t if a subterm of t is a substitution instance of s.
It is used e.g. in the Knuth-Bendix completion algorithm.
Properties
- Encompassment is a preorder, i.e. reflexive and transitive, but not anti-symmetric,[note 1], nor total[note 2]
- The corresponding equivalence relation, defined by s~t if s≤t≤s, is equality modulo renaming.
- s ≤ t whenever s is a subterm of t.
- s ≤ t whenever t is a substitution instance of s.
- The union of any well-founded rewrite order R[note 3] with (<) is well-founded, where (<) denotes the irreflexive kernel of (≤).[3] In particular, (<) itself is well-founded.
Notes
- ^ since both f(x) ≤ f(y) and f(y) ≤ f(x) for variable symbols x, y and a function symbol f
- ^ since neither a≤b nor b≤a for distinct constant symbols a, b
- ^ i.e. irreflexive, transitive, and well-founded binary relation R such that sRt implies u[sσ]p R u[tσ]p for all terms s, t, u, each path p of u, and each substitution σ
References
- ^ Gerard Huet (1981). "A Complete Proof of Correctness of the Knuth-Bendix Completion Algorithm". J. Comput. System Sci. 23 (1): 11–21.
- ^ N. Dershowitz, J.-P. Jouannaud (1990). Jan van Leeuwen (ed.). Rewrite Systems. Handbook of Theoretical Computer Science. Vol. B. Elsevier. pp. 243–320. Here:sect.2.1, p.250
- ^ Dershowitz, Jouannaud (1990), sect.5.4, p.278; somewhat sloppy, R is required to be a "terminating rewrite relation" there.
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