/**
   Doubly linked lists of integers. The list is a reference
   on the cell containing the first element.
*/
public class DListInt {
    int element;
    DListInt prec, next;
    /**
       Creates a cell containing <code>p</code>.
    */
    public DListInt(int p) { element = p; }
    /**
       Inserts <code>p</code> in front.
    */
    public DListInt(int p, DListInt s) { // inserts
	this(p);
	if (s != null) {
	    next = s;
	    prec = s.prec;
	    s.prec = this;
	}
    }
    /**
       Inserts the cell <code>head</code> in front of the list.
    */
    public DListInt insert(DListInt head) {
	next = head;
	prec = null;
	if (next != null)
	    next.prec = this;
	return this;
    }
    /**
       Removes the cell <code>head</code> in front of the list
       and returns it.
    */
    public DListInt remove(DListInt head) {
	if (prec != null)  // not the first element in the list
	    prec.next = next;
	else  // the first element in the list
	    head = next;
	if (next != null)
	    next.prec = prec;
	prec = next = null;
	return head;
    }
    /**
       Returns the list ended with a dot.
    */
    public String toString() {
	return "" + element + " " + ((next == null) ? "." : next.toString());
    }
}