import extensions;
import system'routines;

class Operable
{
    Operable = self;

    operate()
        = "delegate implementation";
}

class Delegator
{
    object? Delegate : prop;

    constructor()
    {
        Delegate := nil
    }

    operate()
    {
        // if the object does not support "Operable" message - returns nil
        var operable := Delegate?.Operable \ back(nil);

        if (nil == operable)
        {
            ^ "default implementation"
        }
        else
        {
            ^ operable.operate()
        }
    }
}

public Program()
{
    var delegator := new Delegator();

    new object[]{nil, new Object(), new Operable()}.forEach::(o)
    {
       delegator.Delegate := o;

       Console.printLine(delegator.operate())
    }
}