This is a vocabulary for working with double cell numeric
values. By using two cells we can achieve a greater range
than the standard 32-bit cells. This probably will not work
correctly on a Retro with 64-bit cells unless you make some
small changes (see the `dn:div` in particular).

~~~
'~res var

:dn:h@ (D-n) fetch ;
:dn:l@ (D-n) n:inc fetch ;
:dn:h! (nD-) store ;
:dn:l! (nD-) n:inc store ;

:dn:new (-D) here #0 dup comma comma ;

:dn:lows  (DD-nn) [ dn:l@ ] bi@ ;
:dn:highs (DD-nn) [ dn:h@ ] bi@ ;

:dn:make (nn-D)  here [ swap comma comma ] dip ;
:dn:put fetch-next n:put sp fetch n:put nl ;

:dn:add (DD-D)
  dn:new !~res
  dup-pair dn:lows n:add @~res dn:l!
  over @~res dn:lows gt? n:abs [ dn:highs n:add ] dip n:add
  @~res dn:h!
  @~res ;

:dn:sub (DD-D)
  dn:new !~res
  dup-pair dn:lows n:sub @~res dn:l!
  over @~res dn:lows lt? n:abs [ dn:highs n:sub ] dip n:sub
  @~res dn:h!
  @~res ;

:dn:mul (DD-D)
  dn:new !~res
  dup-pair dn:lows n:mul @~res dn:l!
  @~res dn:l@ n:negative? n:abs
  [ dup-pair
      [ dn:h@ ] [ dn:l@ ] bi* n:mul
    [ [ dn:l@ ] [ dn:h@ ] bi* n:mul ] dip n:add
  ] dip n:add
  @~res dn:h!
  @~res ;

:dn:div (DD-D)
  dn:new !~res
  dup-pair dn:lows n:div @~res dn:l!
  dup-pair
    [ dn:h@ ] [ dn:l@ ] bi* n:div #-1 shift
  [ [ dn:l@ ] [ dn:h@ ] bi* n:mod #31 shift ] dip n:add
  @~res dn:h!
  @~res ;
~~~