Selects the specified display as the current input display. This
    causes that display to echo debugger input (which appears in the
    PROMPT display). The display specified must be an output display.

    If you do not specify a display, the current input display
    is unselected and debugger input is not echoed to any display
    (debugger input appears only in the PROMPT display). By default,
    no display has the input attribute.