Tube Indexing and Symsets
The most probable trajectory for a vertex muon striking a gap-1 logical tube is to continue on a path of equal dx/dz (vertical tubes) or dy/dz (horizontal tubes).
Tubes w/ the same dx/dz (or dy/dz) get the same index.
Tubes are collected symmetrically about the central tubes (in index space) to form the symsets (one for each tube in the first gap).