I don't know correct solution of my problem.

Here my example:
---begin 1.ml---
#!/usr/bin/env ocamltop

class virtual figure () =
  object
    method virtual to_string: string
  end

class triangle () =
  object
    inherit figure ()
    method private to_string = "Triangle"
    method m1 = Random.float 5.0
  end

class circle () =
  object
    inherit figure ()
    method private to_string = "Circle"
    method m2 = Random.int 6
  end

class screen () =
  object
    val mutable figures = []
    method add_figure f =
      figures <- (f#to_string ) :: figures
  end

let main () =
  let s1 = new screen () in 
  s1#add_figure (new circle ());
  s1#add_figure (new triangle ())

let () = main ()

--end---
[inv@localhost ~]$ ./1.ml
File "./1.ml", line 22, characters 5-121:
Some type variables are unbound in this type:
  class screen :
    unit ->
    object
      val mutable figures : 'a list
      method add_figure : < to_string : 'a; .. > -> unit
    end
The method add_figure has type < to_string : 'a; .. > -> unit where ..
is unbound

I understand the problem, but i don't know solution, help, please.
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