VT100: BACKSPACE            VT200: F12
    DEFINE_KEY Function: BACK_TAB

    Moves the cursor, depending on the type of screen. On a formatted
    screen, the cursor moves one of the following ways, depending on
    the cursor's location when you press this key:

    o  If the cursor is in a field, but not at the first position of
       the field, it moves to the beginning of the unprotected field
       that it is in.

    o  If the cursor is in the first position of a field, it moves
       to the beginning of the preceding unprotected field. If the
       cursor is in the first position of the first unprotected
       field, the cursor moves to the first position of the last
       unprotected field on the screen.

    On an unformatted screen, the cursor returns to the first
    position on the screen.