/**
     * Assume there is a private field called _fac that is an IListFactory.
     * The constructor should have an IListFactory as a parameter.
     */
    public IList removeRange(Comparable keyLo, Comparable keyHi) {
        if (keyLo.compareTo(keyHi) > 0) {
            throw new IllegalArgumentException(keyLo + " > " + keyHi);
        }

        final DictionaryPair low = new DictionaryPair(keyLo, null);
        final DictionaryPair hi = new DictionaryPair(keyHi, null);

        /**
         * Uses an anonymous BiTree visitor with a list accumulator to store the
         * elements that are being removed as the tree _bt is being traversed.
         * There are three cases to consider:
         * root < keyLo: because of BST, the left subtree of _bt is out of range;
         * thus only the right subtree of _bt need to be considered
         * keyHi < root: because of BST, the right subtree of _bt is out of range;
         * thus only the left subtree of _bt need to be considered
         * keyLo <= root <= keyHi: recur on both the left and right subtree of
         * _bt and remove the root node as well.
         */
        return (IList)_bt.execute(new IVisitor() {
            public Object emptyCase(BiTree h, Object accList) {
                return accList;
            }

            public Object nonEmptyCase(BiTree h, Object accList) {
                if (((Comparable)h.getRootDat()).compareTo(low) < 0) {
                    return h.getRightSubTree().execute(this, accList);
                }
                else if (((Comparable)h.getRootDat()).compareTo(hi) > 0) {
                    return h.getLeftSubTree().execute(this, accList);
                }
                else {
                    accList = h.getLeftSubTree().execute(this, accList);
                    accList = h.getRightSubTree().execute(this, accList);
                    accList = _fac.makeNEList(h.getRootDat(), (IList)accList);
                    h.execute(_deleter, h.getRootDat());
                    return accList;
                }
            }
        }, _fac.makeEmptyList());
    }