class virtual person name surname gender =
object
    val mutable name = (name: string)
    val mutable surname = (surname: string)
    val mutable gender = (gender: char)
    method virtual printNameSurname : unit
    method virtual changeSurname newSurname: unit
end;;

(*abstract class for person*)

class male name surname =
object
    inherit person name surname 'm'
    val mutable name = (name: string)
    val mutable surname = (surname: string)
    method printNameSurname = Print.printf "name %s surname %s gender %c" name surname gender
    method changeSurname newSurname = surname <- newSurname
end;;

(*default value for the gender of the male subclass is 'm' *)