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  Name:     isextended() - see if BIOS supports enhanced keyboard
  Usage:    <logical> = isextended()
  Params:   none
  Returns:  logical .T. if the BIOS supports the extended keyboard
            functions, .F. if not.

 ---------------------------------- Example ---------------------------------

                 if (isextended())
                      ? "This machine can handle an enhanced"
                      ? "keyboard"
                 endif

  Note:     Just because the BIOS supports the extended keyboard
            function call does not mean that the computer has
            an enhanced keyboard attached to the computer. Also,
            some clone BIOS handle the enhanced keyboard but still
            return .F. with isextended(). This function is based
            on the specifications in the IBM Technical Reference
            manuals for the PS/2 line of computers. You can do
            a down and dirty check if the computer supports the
            enhanced keyboard by assigning a string to function
            key 41 (F11), then keyboard the inkey() value of F11
            followed by a return. Then enter into a wait state (like
            WAIT to MEMVAR), which will immediately terminate
            because you keyboarded some input. If the returned
            input equals that of the string assigned to key F11,
            then you can assume that the BIOS does handle the
            extended function keys. You can also check by seeing
            if the function keys F11 or F12 return an inkey() value.