Some years ago the British magazine, Gramophone published a list of the world’s 20 top choirs. Most of these were from the UK, the rest from Europe. No American choirs made the list, not even Chanticleer. If that list were to be redone for 2018, the Boston-based group, Skylark would be a serious contender. This Passiontide program was conceived by Skylark’s artistic director, Matthew Guard for a Skylark concert in the Holy Week Festival run by Tenebrae, one of the UK’s most exciting choirs. That Tenebrae is an inspiration for Skylark and that theirs is a cordial relationship says much.