@Serializable
enum class Grade (val mark: Int){
    A(5),
    B(4),
    C(3),
    F(2);

    companion object {
        val list = listOf(A, B, C, F)
    }
}

@Serializable
class GradeInfo(
    val studentId: StudentId,
    val grade: Grade?
)

@Serializable
class Lesson(
    val name: String,
    val students: Array<GradeInfo> = emptyArray()
) {
    fun addStudent(studentId: StudentId) =
        Lesson(
            name,
            students + GradeInfo (studentId, null)
        )
}