5.2.2.9 prevpage="..." and nextpage="..." attributes

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5.2.2.9 `prevpage="..."` and `nextpage="..."` attributes

The `prevpage` and `nextpage` attributes respectively select the previous and next pages of an `<al-for>` `ListIterator` (5.3.8.8).

An attribute value must be supplied that specifies the name of the iterator.

The `name` (5.2.2.5) attribute is generated as follows:

    >>> import albatross
    >>> ctx = albatross.SimpleContext('.')
    >>> albatross.Template(ctx, '<magic>', '''
    ... <al-input type="image" nextpage="i" srcicons/right.gif" border="0" whitespace>
    ... ''').to_html(ctx)
    >>> ctx.flush_content()
    <input type="image" srcicons/right.gif" border="0" name="nextpage,i" />

When merging the browser request the `NamespaceMixin.set_value()` (7.6) method looks for field names that contain commas. These names are split into operation, iterator, and optional value then the `set_backdoor()` method of the identified iterator is invoked.

During request merging the above example will execute code equivalent to the following.

    ctx.locals.i.set_backdoor('nextpage', 'nextpage,i')