  The Burrows-Wheeler Transform (BWT) is an important technique both in data
compression and in the design of compact indexing data structures. It has been
generalized from single strings to collections of strings and some classes of
labeled directed graphs, such as tries and de Bruijn graphs. The BWTs of
repetitive datasets are often compressible using run-length compression, but
recently Baier (CPM 2018) described how they could be even further compressed
using an idea he called tunneling. In this paper we show that tunneled BWTs can
still be used for indexing and extend tunneling to the BWTs of Wheeler graphs,
a framework that includes all the generalizations mentioned above.
