A shortcut key represents a way of accessing a menu using the keyboard. Each
item of a menu can have a letter underlined, indicating that it can be selected
by holding down the Alt key and pressing the letter (or certain other keys).
The components of subsequent sub-menus can then be selected by pressing further
keys corresponding to underlined letters in the menus. The whole process of
underlining the letters and responding to the keyboard is controlled by merely
prefixing the letter in the menu item by an '&' (see %mn and %sm).