Pipeline

Pipeline.prototype.exec = function () {
  var fns = this.fns.slice(0);
  var args = Array.prototype.slice.call(arguments, 0);
  var done = args.length && typeof args[args.length - 1] === 'function' ? args.pop() : function () {};

  if (!args.length && fns.length && fns[0].length > 1) {
    args.push({});
    while (args.length < fns[0].length - 1) { args.push(null); }
  }

  var next = function (err) {
    if (!err && fns.length) {
      fns.shift().apply(null, args.concat([next]));
    } else {
      done.apply(null, [err].concat(args));
    }
  };

  setTimeout(next, 0);
  return this;
};