Discussion of ‘Detecting possibly frequent change-points: Wild Binary Segmentation 2 and steepest-drop model selection’

We congratulate the author for this interesting paper which introduces a novel method for the data segmentation problem that works well in a classical change point setting as well as in a frequent jump situation. Most notably, the paper introduces a new model selection step based on finding the ‘steepest drop to low levels’ (SDLL). Since the new model selection requires a complete (or at least relatively deep) solution path ordering the change point candidates according to some measure of importance, a new recursive variant of the Wild Binary Segmentation (Fryzlewicz in Ann Stat 42:2243–2281, 2014, WBS) named WBS2, has been proposed for candidate generation.

Graph Path Orderings

We define well-founded rewrite orderings on graphs and show that they can be used to show termination of a set of graph rewrite rules by verifying all their cyclic extensions. We then introduce the graph path ordering inspired by the recursive path ordering on terms and show that it is a well-founded rewrite ordering on graphs for which checking termination of a finite set of graph rewrite rules is decidable. Our ordering applies to arbitrary finite, directed, labeled, ordered multigraphs, hence provides a building block for rewriting with graphs, which should impact the many areas in which computations take place on graphs.

TERMINATION OF ABSTRACT REDUCTION SYSTEMS

We present a general theorem capturing conditions required for the termination of abstract reduction systems. We show that our theorem generalises another similar general theorem about termination of such systems. We apply our theorem to give interesting proofs of termination for typed combinatory logic. Thus, our method can handle most path-orderings in the literature as well as the reducibility method typically used for typed combinators. Finally we show how our theorem can be used to prove termination for incrementally defined rewrite systems, including an incremental general path ordering. All proofs have been formally machine-checked in Isabelle/HOL.