This assignment comes in two parts.
The second part builds upon a correct solution to the first part.

PART ONE

Make a Reverse Polish Notation Calculator

If you don't know about Reverse Polish Notation, don't worry. It is very
easy to understand, and even the wikipedia entry for it is understandable.

  This program should have an interactive loop:
    program asks for a formula
    user types a formula, as a single string
      if the string is "stop" then exit
    program evaluates that formula
    program prints the result

  Here's a small sample run showing the sort of thing that should work:

  formula: 1 2 3 4 5 + + + +
  result is 15

  formula: 12.3 45.6 +
  result is 57.9

  formula: 1 2 + 3 + 4 + 5 + 10 /
  result is 1.5

  formula: 1 2 * 3 4 * + 5 +
  result is 19

  formula: 1 2 + 3 4 + * 5 *
  result is 105

  formula: stop

Your program should be crash-proof and fool-proof. That means that it should 
continue running no matter what bad inputs the user provides, and it should
never print an invalid result, the results are only allowed to be proper
numbers. Check for all possible problems.



PART TWO

Make the Reverse Polish Notation calculator work with variables

To do this, we introduce a new operator, ->. This does not behave
like a normal operator. Immediately following the -> there must
be a variable name. Whatever is on top of the stack becomes the
value of that variable, but it isn't removed, it the value remains
on the top of the stack.

Whenever a name appears anywhere else in a formula, the value of
that variable is pushed onto the stack.

An example run:

  formula: 3.14159 -> pi
  result is 3.14159

  formula: 10 -> r
  result is 10

  formula: pi r r * * -> area
  result is 314.159

  formula: stop

That's it.