@Christophe To my understanding, the term "class" in a class diagram represents the principle of object-oriented programming. You cannot simply say that the model must be separated from the implementation. Rather, the model must be precisely transferable to the respective implementation and vice versa. The remaining question is: how do I represent the following implementation in a UML class diagram?

class LeaseAgreement{
    public Person tenant{
        get{
            return tenant;
        }
        set{
            tenant = value;
        }
    }

}

class Person{
    public string name{get{return name;}set{name=value;}}
    public string lastname{get{return lastname;}set{lastname=value;}}
}