Tests of the (slightly) customized collections in ``plib.stdlib.coll``,
to ensure that the ``next`` methods retrieve the appropriate "next"
member of the collection.

    >>> from plib.stdlib import coll

The ``fifo`` collection should retrieve the "oldest" item.

    >>> f = coll.fifo()
    >>> f.append(1)
    >>> f
    deque([1])
    >>> f.append(2)
    >>> f
    deque([1, 2])
    >>> f.next()
    1
    >>> f.next()
    2

The ``stack`` collection should retrieve the most recent item.

    >>> s = coll.stack()
    >>> s.append(1)
    >>> s
    [1]
    >>> s.append(2)
    >>> s
    [1, 2]
    >>> s.next()
    2
    >>> s.next()
    1
