VT100: Ctrl/V               VT200: EXT + F17
    DEFINE_KEY Function: DISPLAY_ATTRIBUTES

    Enables and disables the visible attribute mode. This mode of
    operation forces display of the attribute characters (that
    is, the characters at the start of a field that indicate the
    display and data type of that field). In IBM 3270 model terminal
    emulation (TN3270), you can use the DSP ATT function to debug
    application programs.