text: var x = [1,2,3];
  var y = x.concat(4,5,6);
  var __result1 = y.toString();
  var __expect1 = "1,2,3,4,5,6";
  var __result2 = y.length;
  var __expect2 = 6;
  var __result3 = x.toString();
  var __expect3 = "1,2,3";
  var __result4 = x.length;
  var __expect4 = 3;
  
  var arr1 = [0,1];
  var arr2 = [2,3];
  var arr3 = arr1.concat(arr2);
  var __result5 = arr3[0];
  var __expect5 = 0;
  var __result6 = arr3[1];
  var __expect6 = 1;
  var __result7 = arr3[2];
  var __expect7 = 2;
  var __result8 = arr3[3];
  var __expect8 = 3;
  var __result9 = arr3[4];
  var __expect9 = undefined;
  