code: function readWith1ArgumentCb(opts, cb) {
    return read(opts, function(err, a, b) {
      cb(err, a);
    });
  }
docs: without this, we'd get: readP(...).then(cb) where callback will receive an one array arguments of the original arguments to make bluebird passes back the single argument without the array wrapper, we make the callback only take one argument (excluding the mandatory first error argument)