“If you’ve come expecting to see Madame Butterfly Part II, you’re in for a surprise,” Nashville Opera artistic director John Hoomes said before a performance of David Lang’s The Difficulty of Crossing a Field. He wasn’t kidding. Lang’s post-minimalist masterpiece is an opera-theater hybrid, a surreal work that’s been described as a cross between Gone With the Wind and The Twilight Zone. The simple plot involves an antebellum planter named Williamson, who walks across a field and vanishes. Hoomes and his cast of actors and opera singers approached the work as a cerebral puzzle piece, a mystery intended to inspire discussion and debate. A quartet of musicians from the Alias Chamber Ensemble played Lang’s pulsating score with flexibility and sensitivity.