Certification Problem

Input (COPS 806)

The rewrite relation of the following conditional TRS is considered.

a a | bx, cx
b d | dx, ex
c d | dx, ex

Property / Task

Prove or disprove confluence.

Answer / Result

Yes.

Proof (by ConCon @ CoCo 2020)

1 Removal of Infeasible Rules

We may safely remove rules with infeasible conditions. They do not influence the rewrite relation in any way.

1.1 Rules with Infeasible Conditions

1.2 Quasi-reductive SDTRS where all CCPs are joinable

The given strongly deterministic oriented 3-CTRS is quasi-reductive and all CCPs are joinable.

1.2.1 Quasi-Reductive CTRS

The given CTRS is quasi-reductive

1.2.1.1 Unraveling

To prove that the CTRS is quasi-reductive, we show termination of the following unraveled system.

For aabxcx we get

1.2.1.1.1 Bounds

The given TRS is match-(raise)-bounded by 4. This is shown by the following automaton. The automaton is closed under rewriting as it is state-compatible w.r.t. the following relation.
6 » 13
6 » 9
6 » 23
6 » 20
6 » 14
6 » 10
6 » 21
6 » 6
14 » 21
14 » 14
10 » 14
10 » 21
10 » 10
21 » 21
13 » 23
13 » 20
13 » 13
9 » 13
9 » 23
9 » 20
9 » 9
23 » 23
20 » 23
20 » 20

1.2.2 All CCPs are joinable

A CCP is joinable if it is context-joinable, infeasible, or unfeasible.