static print(arr) {
    if (arr.length == 0) {
      return '';
    }

    let s = '(';

    for (let i = 0; i < arr.length; i++) {
      s += arr[i].id ? arr[i].id + ', ' : arr[i] + ', ';
    }

    s = s.substring(0, s.length - 2);
    return s + ')';
  }
Returns a string representation of an array. If the array contains objects with an id property, the id property is printed for each of the elements.

@static
@param {Object[]} arr An array.
@param {*} arr[].id If the array contains an object with the property 'id', the properties value is printed. Else, the array elements value is printend.
@returns {String} A string representation of the array.