module DATA is

!library
!update "2021-b"

------------------------------------------------------------------------------

type ADR is
   range 1...3 of NAT
end type

------------------------------------------------------------------------------

type MSG is
   range 1...3 of NAT
   with ==, >, >=
end type

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function N: MSG is
   return 3 of MSG
end function

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function <+> (X, Y: MSG): MSG is
   var RESULT: NAT in
      RESULT := NAT (X) + NAT (Y);
      assert RESULT <= 2 * NAT (N);
      if RESULT > NAT (N) then
         RESULT -= NAT (N)
      end if;
      return MSG (RESULT)
   end var
end function

------------------------------------------------------------------------------

function <-> (X, Y: MSG): MSG is
   var RESULT: NAT in
      RESULT := NAT (X);
      if RESULT <= NAT (Y) then
         RESULT += NAT (N)
      end if;
      return MSG (RESULT - NAT (Y))
   end var
end function

------------------------------------------------------------------------------

type TYPEMSG is
   FIRST, SECOND
end type

-------------------------------------------------------------------------------

end module