Function callback

test/queue-test.js:16–22

(error, one, two, three)

      .await(callback);

  function callback(error, one, two, three) {
    test.equal(error, null);
    test.ok(one.size > 0);
    test.ok(two.size > 0);
    test.ok(three.size > 0);
    test.end();
  }
});

tape("if the concurrency is invalid, an Error is thrown", function(test) {