INT 12 - Back&Forth v1.62+ - API
        AX = FFFEh
        CX = FFFEh
        BX = function
            00h installation check
                Return: AX = 0001h installed
                             else  not loaded
            02h build program ID list
                ES:DI -> buffer of at least 100 bytes, to be filled with words
                Return: AX = number of programs defined
                        ES:DI buffer filled with AX words
            03h switch to specified task (task need not be open yet)
                DX = two-letter program ID
                Return: AX = 0000h if task undefined
            04h ???
            05h ???
            06h get version (undoc)
                Return: AX = version * 100 (v1.71 = 00ABh)
            07h ???
            08h get open tasks (undoc)
                ES:DI -> task info buffer (see below)
                Return: AX = number of open tasks (max 20)
            09h ???
Program: Back & Forth is a task switcher by Progressive Solutions, Inc.
SeeAlso: INT 11/AX=FFFEh
Index:  installation check;BACK&FORTH

Format of task info buffer:
Offset  Size    Description
 00h 21 BYTEs   ASCIZ task name
 15h    BYTE    hotkey shift state (as for INT 16/AH=02h)
 16h    WORD    hotkey scan code
 18h    WORD    program ID
Index:  hotkeys;Back&Forth

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