~~~
:double:var (nns-)
  d:create swap , , ;

:double:fetch (a-nn)
  fetch-next swap fetch ;

:double:store (nna-)
  &swap dip store-next store ;

:double:const (nns-)
  double:var &double:fetch does ;

:double:swap (nnmm-mmnn)
  rot push rot pop ;

:double:dip (mnq-mn) rot rot push push call pop pop ;
:double:sip (mnq-mn) &dup-pair dip double:dip ;

'interface/double.retro 'double:var   d:set-source
'interface/double.retro 'double:fetch d:set-source
'interface/double.retro 'double:store d:set-source
'interface/double.retro 'double:const d:set-source
'interface/double.retro 'double:swap  d:set-source
'interface/double.retro 'double:sip   d:set-source
'interface/double.retro 'double:dip   d:set-source
~~~