### code.ml (ocaml)
class virtual number: object ('a)
  method virtual value: 'b
  method virtual plus: 'a -> 'a
end = object
  method virtual value: 'b
  method virtual plus: 'a -> 'a
end

### error.txt

Error: The class type
         object method virtual plus : 'a -> 'a method virtual value : 'b end
       is not matched by the class type
         object ('a)
           method virtual plus : 'a -> 'a
           method virtual value : 'b
         end
       The method plus has type 'a -> 'a but is expected to have type
         (< plus : 'b; value : 'd. 'd; .. > as 'c) -> 'c as 'b