spans: Extend cells over multiple rows and/or columns
Description
A cell with rowspan of 2 covers the cell directly below it. A cell with
colspan of 2 covers the cell directly to its right. A cell with rowspan of 2
and colspan of 2 covers a 2 x 2 square, hiding three other cells.
Usage
rowspan(ht)
rowspan(ht) <- value
set_rowspan(ht, row, col, value )
map_rowspan(ht, row, col, fn)
colspan(ht)
colspan(ht) <- value
set_colspan(ht, row, col, value )
map_colspan(ht, row, col, fn)