groups for types? interface in Nim?

[kobi]

I would like to have a way to group certain types together, for example, for 
the usage of interface in other languages. 
    
    
    type ICanFly = ref object of RootRef
    type Bird = ref object of ICanFly
    type Fly = ref object of ICanFly
    
    var flies :seq[ICanFly] = @[ Bird(), Fly()]
    
    
    Run

but in this case, ICanFly can be instantiated. (final pragma didn't seem to 
work!) 
    
    
     flies.add ICanFly() 
    
    Run

a plus would be to avoid the inheritance mechanism:

  * I simply want to group them together.
  * Can a Bird extend more than one interface, in terms of other languages, say 
ICanFly, and IHaveWings