He wants to be able to pass any Proc to his method, and any arguments supplied to the method are yielded onto target. A rough Objective-C equivalent would be something like this:

    void animate(void (^block)(va_list), ...)
    {
        va_list args;
        va_start(args, block);
        block(args);
        va_end(args);
    }

    void (^first)(va_list) = ^(va_list args) {
        NSLog(@"%@", va_arg(args, id));
    };

    void (^second)(va_list) = ^(va_list args) {
        NSLog(@"%@", va_arg(args, id));
        NSLog(@"%@", va_arg(args, id));
    };

    animate(first, @"Hello World");
    animate(second, @"Hello", [[NSNumber alloc] initWithInt:10]);

I would like to see the real-world code too. The frameworks in question revolve around Obj-C-isms, so to see something that completely deviates from those patterns will be interesting.