> class dtree =
>   object
>     inherit [#debug] ctree
>   end
> 
> But the compiler gives me the error:
> 
> Some type variables are unbound in this type:
>   class dtree :
>     object val mutable content : (#debug as 'a) tree method get : 'a
> tree end
> The method get has type #debug tree where .. is unbound
> make: *** [test.cmo] Error 2   
> 
Here is a simple solution: just write your class as:

class ['a] dtree =
  object
    constraint 'a = #debug
    inherit ['a] ctree
  end

Indeed, no type variable can stay unbounded (hence the ['a]) in a
class definition. #debug (equivalent to < debug : unit; .. >) is an
abbreviation that hides a type variable (the elipsis: ..).

-- 
Pierre.