Path ordering (term rewriting)

In theoretical computer science, in particular in term rewriting, a path ordering is a well-founded strict total order (>) on the set of all terms such that

f(...) > g(s₁,...,s_n)   if   f ^.> g   and   f(...) > s_i for i=1,...,n,

where (^.>) is a user-given total precedence order on the set of all function symbols.

Intuitively, a term f(...) is bigger than any term g(...) built from terms s_i smaller than f(...) using a lower-precedence root symbol g. In particular, by structural induction, a term f(...) is bigger than any term containing only symbols smaller than f.

A path ordering is often used as reduction ordering in term rewriting, in particular in the Knuth–Bendix completion algorithm. As an example, a term rewriting system for "multiplying out" mathematical expressions could contain a rule x*(y+z) → (x*y) + (x*z). In order to prove termination, a reduction ordering (>) must be found with respect to which the term x*(y+z) is greater than the term (x*y)+(x*z). This is not trivial, since the former term contains both less function symbols and less variables than the latter. However, setting the precedence (*) ^.> (+), a path ordering can be used, since both x*(y+z) > x*y and x*(y+z) > x*z is easy to achieve.

Given two terms s and t, with a root symbol f and g, respectively, to decide their relation their root symbols are compared first.

• If f <^. g, then s can dominate t only if one of s's subterms does.
• If f ^.> g, then s dominates t if s dominates each of t's subterms.
• If f = g, then the immediate subterms of s and t need to be compared recursively. Depending on the particular method, different variations of path orderings exist.^[1][2]

The latter variations include:

• the multiset path ordering (mpo), originally called recursive path ordering (rpo)^[3]
• the lexicographic path ordering (lpo)^[4]
• a combination of mpo and lpo, called recursive path ordering by Dershowitz, Jouannaud (1990)^[5][6][7]

Dershowitz, Okada (1988) list more variants, and relate them to Ackermann's system of ordinals.

Formal definitions

The multiset path ordering (>) can be defined as follows:^[8]

s = f(s1,...,sm) > t = g(t1,...,tn)									if
f	^.>	g	and	s	>	tj	for each	j∈{1,...,n},	or
				si	≥	t	for some	i∈{1,...,m},	or
f	=	g	and	{ s1,...,sm } >> { t1,...,tn }

where

• (≥) denotes the reflexive closure of the mpo (>),
• { s₁,...,s_m } denotes the multiset of s’s subterms, similar for t, and
• (>>) denotes the multiset extension of (>), defined by { s₁,...,s_m } >> { t₁,...,t_n } if { t₁,...,t_n } can be obtained from { s₁,...,s_m }
  • by deleting at least one element, or
  • by replacing an element by a multiset of strictly smaller (w.r.t. the mpo) elements.^[9]

More generally, an order functional is a function O mapping an ordering to another one, and satisfying the following properties:^[10]

• If (>) is transitive, then so is O(>).
• If (>) is irreflexive, then so is O(>).
• If s > t, then f(...,s,...) O(>) f(...,t,...).
• O is continuous on relations, i.e. if R₀, R₁, R₂, R₃, ... is an infinite sequence of relations, then O(∪∞
  i=0 R_i) = ∪∞
  i=0 O(R_i).

The multiset extension, mapping (>) above to (>>) above is one example of an order functional: (>>)=O(>). Another order functional is the lexicographic extension, leading to the lexicographic path ordering.

References

1. Nachum Dershowitz, Jean-Pierre Jouannaud (1990). Jan van Leeuwen, ed. Rewrite Systems. Handbook of Theoretical Computer Science B. Elsevier. pp. 243–320. Here: sect.5.3, p.275
2. Gerard Huet (May 1986). Formal Structures for Computation and Deduction. International Summer School on Logic of Programming and Calculi of Discrete Design. Here: chapter 4, p.55-64
3. N. Dershowitz (1982). "Orderings for Term-Rewriting Systems" (PDF). Theoret. Comput. Sci. 17 (3): 279–301.
4. S. Kamin, J.-J. Levy (1980). Two Generalizations of the Recursive Path Ordering (Technical report). Univ. of Illinois, Urbana/IL.
5. Kamin, Levy (1980)
6. N. Dershowitz, M. Okada (1988). "Proof-Theoretic Techniques for Term Rewriting Theory". Proc. 3rd IEEE Symp. on Logic in Computer Science (PDF). pp. 104–111.
7. Mitsuhiro Okada, Adam Steele (1988). "Ordering Structures and the Knuth–Bendix Completion Algorithm". Proc. of the Allerton Conf. on Communication, Control, and Computing.
8. Huet (1986), sect.4.3, def.1, p.57
9. Huet (1986), sect.4.1.3, p.56
10. Huet (1986), sect.4.3, p. 58