// Returns an Iterator<BLT> that generates this tree
    // and all its subtrees, in preorder left-to-right order.

    public Iterator<BLT> iterator () {
	ArrayList<BLT> a = new ArrayList<BLT>();
	enumerateSubtrees (this, a);

        // Returning a.iterator() would almost be correct,
        // but the problem statement requires the remove()
        // method to throw a certain exception.

	return new IteratorWithNoRemove<BLT>(a.iterator());
    }

    // Given a BLT t and an ArrayList,
    // adds the subtrees of t to the ArrayList.

    private void enumerateSubtrees (BLT t, ArrayList<BLT> a) {
	a.add(t);
	if (! (t.isLeaf())) {
	    enumerateSubtrees(t.left(), a);
	    enumerateSubtrees(t.right(), a);
	}
    }

    // IteratorWithNoRemove encapsulates an Iterator
    // and hides its remove() method.

    private static class IteratorWithNoRemove<T> implements Iterator<T> {
	// the encapsulated Iterator
	private Iterator<T> it;
	IteratorWithNoRemove (Iterator<T> it) { this.it = it; }

	// the hasNext() method delegates to this.it

	public boolean hasNext() { return it.hasNext(); }

	// the next() method delegates to this.it

	public T next() { return it.next(); }

	// but the remove() method throws an exception.

	public void remove() { throw new UnsupportedOperationException(); }
    }