Copy these files to your cs323/hw2/, and edit PRedBlackBST.java
as directed by the TODO comments.

You want to modify class PRedBlackBST, which extends PBST (mostly seen
last time) so that PRedBlackBST is a persistent left-leaning red-black
with insertion (insertion is the "put" method).

You should mimic the "ephemeral" methods from the book, for example:

   http://mathcs.emory.edu/~cs323000/book/RedBlackLiteBST.java

The basic trick is that whenever the book code would modify a Node,
you instead need to create a new Node, taking its place.

For 10 points extra credit, you can also try to implement a
stack-based tree iterator, which is somewhat more efficient than the
iterator I've defined in PBST. Let me know by email if you succeed
with this part.

This is due 9pm Tuesday 9/21 (written part 5pm).

The turnin procedure is not determined yet, but possibly we'll try the
"turnin" script again, I'm told it should work now.  Stay tuned.

There ought to be a better test harness.  For now, you want to
eliminate all the potential "check" error messages when you do this:

    javac *.java
    java Driver