The Basic Outline of a Tree Walking Interpreter
  ===============================================


1.) Start with a string in our language.

"(&& (<= 23 25) (! (>= 100 300)))"


2.) Tokenizer.java produces a list of tokens.

[ (, &&, (, <=, 23, 25, ), (, !, (, >=, 100, 300, ), ), ) ]


3.) Tree.java and BuildTree.java build an abstract syntax tree.

                && 
               /  \
              /    \
            <=      !
           /  \     |
         23   25    |
                    >=
                   /  \
                100    300


4.) Evaluate.java traverses the AST and yields a Value.java result.

true


5.) PrettyPrinter.java traverses the AST and produces an indented string.

"(&& (<= 23 25)
     (! (>= 100 300)))"


6.) AST2Infix.java traverses the AST and produces an infix notation string.

"(23 <= 25) && !(100 >= 300)"


7.) Tree2Dot.java traverses the AST and produces a dot file that 
    can be converted by dot.exe into a png image file of the AST.