class Pair<alpha, beta> {                
    pub. alpha a;                                                          
    pub. beta b;                                                           
}                                                                          
                                                                           
class List<gamma> ext. Pair<alpha :- gamma, beta :- List<gamma :- gamma>> {}             
                                                                           
class Tree<delta> 
{ Pair<alpha :- delta, beta :- List<gamma :- Tree<delta :- delta>>> a; }
                                                                           
class Printable {                                                          
    pr(Txt x, Int y) {}                                                    
}                                                                          
                                                                           
class Empty {}                                                             
                                                                           
fun isz(&Empty) -> Bool;                                                   
                                                                           
main {                                                                     
    Tree<delta :- Printable> tr, Bool bo;                                  
    var Bool bo2;                                                          
    var Bool bo3;                                                          
    Tree<delta :- Printable>.new(tr);                                      
    bo := true;                                                            
    bo2 := bo;                                                             
    bo3 := bo2;                                                            
    end bo3;                                                               
    end bo2;                                                               
}