All operas are about more than just the music. In the case of composer Tansy Davies and librettist Nick Drake’s new creation Cave, however, the music, word, message and setting all possess such significance that the work would have been all the poorer had any of them not received the attention that they do. This is the second time that Davies and Drake have collaborated, following Between Worlds, which was produced by English National Opera and staged at the Barbican Theatre in...