Error: Some type variables are unbound in this type:
         class normalization_cmd :
           unit ->
           object
             val normalize : MapsSets.StringMap.key Subcommand.flag
             method get_normalization :
               < get_bl : float; .. > Gtree.gtree -> float
             method specl : (string * Arg.spec * string) list
           end
       The method get_normalization has type
         (< get_bl : float; .. > as 'a) Gtree.gtree -> float
       where 'a is unbound