On treeings arising from HNN extensions

For certain HNN extensions including Baumslag-Solitar groups, a treeing is constructed from their certain probability-measure-preserving actions. This is a treeing of a quotient groupoid of the translation groupoid associated with their actions. As its application, for some of those HNN extensions, we show that the kernel of the modular homomorphism is measure equivalent to the direct product of the free group of infinite rank and Z.