code: function contains(item, equalityComparer) {
    equalityComparer = typeof equalityComparer === 'function' ? equalityComparer : util.defaultEqualityComparer;
  
    for (let i of this) {
      if (equalityComparer(i, item)) return true;
    }
  
    return false;
  }
docs: Determines whether a sequence contains a specified element by using the default equality comparer.
  
  @param item The item which you want to check.
  @param {(Function)} equalityComparer The equality comparer.
  equalityComparer: (item1, item2) -> Boolean