SAMPLE DEVELOPMENT INTRODUCTION

Vision: a calculator for numerical tasks.
    The system can store numbers
    in registers.
    The system can store formulas
    as "subroutines"; these can
    refer to the values in registers.

Use cases for first iteration:

 Store a number in a register:
   The user tells the system they want
   to enter a number into a register.
   The user tells the system the number.
   The system remembers the number.
   The user tells the system what register
   to store the number in.
   The system remembers that the number
   is in the given register.

 Store a formula in a subroutine:
   The user tells the system they want
   to store a formula in a subroutine.
   The user tells the system the name of
   the subroutine.
   The system remembers the name.
   The user gives the system the formula.
   The system remembers that the formula
   has the given name.

 Evaluate a formula.
   The user tells the system they want
   to evaluate a given formula, supplying
   the name.
   The system tells the user the result.