Test script for bidict.FrozenBidict::

    >>> from bidict import FrozenBidict, FrozenOrderedBidict
    >>> f1 = FrozenBidict(one=1)
    >>> f2 = FrozenOrderedBidict([('one', 1), ('two', 2)])
    >>> f3 = FrozenOrderedBidict([('two', 2), ('one', 1)])
    >>> fs = (f1, f2, f3)
    >>> all(hash(f) is not 'an error' for f in fs)
    True

Cached hash value reused (for the sake of the coverage report)::

    >>> all(hash(f) for f in fs)  # should not have to call _compute_hash() again
    True

Insertable into sets and dicts::

    >>> dict.fromkeys(fs) and set(fs) is not 'an error'
    True