The work area and its borders
The work area is surrounded by a border giving the row and column numbers.
When there are multiple windows, there will be several borders.
A cell is a box that can hold data. The cell cursor is the highlighted
rectangle indicating the `active cell'. A region is a rectangular block
of cells. Regions are described by giving coordinates of the upper left
cell and lower right cell.
The appearance of the screen can be changed (see section 6.1 Changing Oleo's appearance).