// For more information see https://aka.ms/fsharp-console-apps

type MarkX() =
    class
        abstract member some:unit-> unit
        default this.some() = 
            printfn "Hello World!"
    end

type SubMark() =
    class
        inherit MarkX()

        override this.some (): unit = 
            printfn "Substract"
    end


let code = SubMark()
code.some()