new_contents: String.format = function () {
      var s = arguments[0];
      for (var i = 0; i < arguments.length - 1; i++) {
          var reg = new RegExp("\\{" + i + "\\}", "gm");
          s = s.replace(reg, arguments[i + 1]);
      }
  
      return s;
  }
  
  String.prototype.endsWith = function (suffix) {
      return (this.substr(this.length - suffix.length) === suffix);
  }
  
  String.prototype.startsWith = function (prefix) {
      return (this.substr(0, prefix.length) === prefix);
  }
  
subject: Update core.js