Some sample expressions for the evaluator
-----------------------------------------

-17

READ

READ+2*3		; should be "parsed" (by hand) as READ+(2*3)

(x=2)+x*(x=7)	; should yield 51, NOT 16

(x=3)+(2*++x)+(3*x++)+(x=1)		; should yield 13

(x--)*READ*--x+(x=y)*(y=3)		; should yield (4 * the input) + 9

-----------------------------------------

To test your evaluator, you would need to hand-translate these examples into 
constructor-call format, e.g. "new Mult( new Plus ( new Lit(2), ... ));"

In order to test a whole slew of examples, you might want to use code
like this:

    // -------------------------

    Expr[] tests = { new Lit(0),
                     new Lit(-17),
                     new READ(),
                     new Plus( new READ(), new Mult(new Lit(2), new Lit(3))),
                     ...
                   };

    for(int i=0; i<tests.length; i++) {
        emit("Test #" + i + ":\t" + tests[i] + "\t==>\t" + tests[i].value());
        globalHash.reset();       // dunno if such a thing exists for the HashMap class ...
    }

    // -------------------------


(Read "System.out.println" for "emit" here, or dump to a textArea if you like.
Also, remember that you should probably re-set the hash table for variables in between 
each two tests ... not that "communication" of variable values between expression evaluations
would be so bad (maybe a feature, not a bug), but it might not jive with your expectations.)

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