Function: table-shorten-cell

Shorten the current cell by N lines by shrinking the cell vertically.
Shortening is done by removing blank lines from the bottom of the cell
and possibly from the top of the cell as well. Therefore, the cell
must have some bottom/top blank lines to be shorten effectively. This
is applicable to all the cells aligned horizontally with the current
one because they are also shortened in order to keep the rectangular
table structure. (fn N)