READ_KEY

       To read a keystroke and return its ASCII equivalent

       If an ESCAPE sequence was produced and  the sequence is
       recognized (e.g. up arrow), then a code is returned.

       key = READ_KEY(Wait)

       Wait  -  The wait flag.  If non-zero, execution is halted until a
                key is struck.  If zero, execution returns immediately and
                a zero is returned if there was no keystroke waiting in the
                keyboard buffer.  If not specified, zero is assumed.

       Returned - The key struck.  The ASCII code for non-escape sequences.
                  Escape sequence equivalents:
                       Up Arrow     --  128
                       Down Arrow   --  130
                       Left Arrow   --  129
                       Right Arrow  --  131
                       Else         --    0
       The return value is a byte value.

       Written by Michael R. Greason, STX, 22 June 1990.
       Rewritten for a SUN workstation.  MRG, STX, 23 August 1990.
       Converted to IDL V5.0   W. Landsman   September 1997

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