Inserts item before the one that was previously obtained with $next()
This can be used after calling $remove or $set() and can be called
multiple times. However, it may fail in some cases.
When calling $insert(one) and then $insert(two) the two item will be
placed after the one item.
This can called before $next() and will insert item before the start
of the box.
This is an optional method, when inserting is not possible it throws an
When inserting is not possible FAIL is returned. Normally the
implementation always works when it is implemented, but in rare cases
it may fail to insert only some items.