This program computes modes of a vacuum magnetic field, ordering them by the efficiency by which they propagate from one toroidal surface to another toroidal surface inside. The method is described in Boozer, Nuclear Fusion 55, 025001 (2015). The main ideas are described on page 12, and some key definitions are given on page 9. Documentation can be found in the user manual here, in the manual/ subdirectory of this repository.