/*
             +----------------------------+                
             |  AList                     |                
             +----------------------------+                
	         | boolean same(Obj that)     |
	         | boolean contains(Obj that) | 
             +----------------------------+                
                         |                    
                        / \                   
                        ---                   
                         |                    
           ---------------------------          
           |                         |          
    +-------------+          +---------------+  
    | MTList      |          | ConsList      |  
    +-------------+          +---------------+  
    +-------------+     +----| ISame fist    | 
                        |    | AList rest    |   
                        |    +---------------+ 
	            		v
            +--------------------------+ 
            | ISame                    |   
	        +--------------------------+
	        | boolean same(ISame that) |
	        +--------------------------+
                        |
                       / \
                       ---
                        |
               --------------------
               |                  |
	+--------------+    +--------------+
	| Book         |    | Song         |
    +--------------+    +--------------+
    | String title |    | String title | 
	| String author|    | int    time  |
	| int    year  |    +--------------+  
    +--------------+    
           
*/  

// to represent a list of objects.
interface AList extends ISame{
  // is this object the same as that object?
  boolean same(ISame that);
  
  // does this list contan the given object?
  boolean contains(ISame that);
}