Hello,

Actually, #debug is an abbrevation for 
 <method debug : unit; ..>
where ".." is a row type variable, you may imagine as universally quantified.
And methods of a class are not allowed to be polymorphic. Only classes are.
So, you have to bound this row variable to a type parameter of the class. 

* So a general solution is :

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

* But if you want only a "debug tree", you may write :

class dtree =
  object
    inherit [debug] ctree
  end

Regards,

Sylvain.