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

    >>> from plib.stdlib import coll

The ``fifo`` collection should retrieve the "oldest" item. Note that
we say ``list(f)`` to display the object because the ``repr`` of the
object itself is not consistent across platforms.

    >>> f = coll.fifo()
    >>> f.append(1)
    >>> list(f)
    [1]
    >>> f.append(2)
    >>> list(f)
    [1, 2]
    >>> f.nextitem()
    1
    >>> f.nextitem()
    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.nextitem()
    2
    >>> s.nextitem()
    1
