def __new__(cls, iterable):
    """Represents an ordered sequence of unique items.

        The `OrderedSet` implements an immutable ordered sequence of
        unique items.

        Args:
            iterable: An `Iterable`. The items of the set. Must not contain
                duplicates.

        Raises:
            ValueError: If the iterable contains duplicates.

        """
    if len(set(iterable)) != len(iterable):
        raise ValueError('The iterable must not contain duplicates.')
    self = tuple.__new__(cls, iterable)
    return self
