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

    public Iterator<BLT> iterator () {
	return new IteratorOfBLTs(this);
    }

    // Instances of this class generate all subtrees of a BLT.

    private static class IteratorOfBLTs implements Iterator<BLT> {

	// a stack that contains
	// all BLTs whose subtrees have not yet been generated

	private Stack<BLT> stk;

	IteratorOfBLTs (BLT t) {
	    this.stk = new Stack<BLT>();
	    this.stk.push(t);
	}

	// Are there more subtrees to be generated?

	public boolean hasNext() { return ! (stk.empty()); }

	// Returns the next subtree to be generated,
	// in preorder, left-to-right order.

	public BLT next() {
	    if (hasNext()) {
		BLT t = stk.pop();
		if (! (t.isLeaf())) {
		    stk.push(t.right());
		    stk.push(t.left());
		}
		return t;
	    }
	    else throw new NoSuchElementException();
	}

	// Always throws an exception.

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