A class that holds the three possible values for a border-before/after on a table-cell,
in the collapsing model. These three values are (for border-before, similar for
border-after):
- normal: common case, when a cell follows the cell before on a same page;
- leading: when the table is broken and the cell appears at the top of a page, in
which case its border must be resolved with the header (or the top of the table)
instead of with the previous cell;
- rest: when a cell is broken over several pages; same as leading but with
conditionality taken into account.