XPd2bin()
  Convert a Double to a binray variable (8 bytes)
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   Function:   XPd2bin()

               In addition to Clipper's l2bin() and alike functions,
               XPd2Bin() and XPbin2d() convert a Clipper Double number into a
               8 bytes long binary character. Added to the library because
               XPd2bin() was needed by the 1-2-3 routines, and XPbin2d()
               was also added as the natural complement of it.

   Syntax:     XPd2Bin(<Numeric>)

   Arguments:  The Numeric expression is converted to a binary
               double precision number (std 8 bytes format).

   Returns:    An 8 bytes string, containing the binary number.