lib/api.js
function(user, identifier, revisionIdentifier, target, out, cb) {
    logger.info('preview revision: ' + identifier + ', ' + revisionIdentifier + ' ' + target);
    deployRevision(user, identifier, revisionIdentifier, target, 'preview', out, function(err) {
      cb(err, {plan: out.getPlan(), ops: out.operations()});
    });
  }
function(user, identifier, revisionIdentifier, target, out, cb) {
    logger.info('preview revision: ' + identifier + ', ' + revisionIdentifier + ' ' + target);
    deployRevision(user, identifier, revisionIdentifier, target, 'preview', out, function(err) {
      cb(err, {plan: out.getPlan(), ops: out.operations()});
    });
  }
preview a system deploy