Selecting pigeonholes
There are two ways to select multiple pigeonholes.
1. If there is an empty slot adjacent to the pigeonholes you want to select, press the left mouse button over the empty slot, then, holding the mouse button down, move the mouse pointer over the pigeonholes you want to select.
2. If there is no adjacent empty slot, hold down the Ctrl key while you move the mouse over the pigeonholes you want to select.
Whichever method you use, moving the mouse over un-selected pigeonholes selects them and moving it back over selected pigeonholes deselects them.