/*
	Interface for elements of D in Jones, Chapter 2
	Written by:  Kim Bruce, 9/16/2002
*/


public interface D {
	// Constants representing nil, true, and false;
	D nil = new Atom("nil");
	D dTrue = new Composite(nil,nil);
	D dFalse = nil;
	
	// create new list with other as head and this as tail
	D cons(D other);  
	
	// return head of this
	D hd();       
	
	// return tail of this    
	D tl();         
	
	// return dTrue if this equals other and dFalse otherwise
	D eq(D other);  

	// return true iff this equals other
	boolean booleq(D other);  
	
        // return true iff this represents a legal list
        boolean isList();
	
        // If this represents a legal list, e.g., (a.(b.(c.nil))), then it
        // returns the list representation of the element, e.g. "(a,b,c)"
	String asList();

}