Inject a bug into

Implement the steps and make the scenarios pass.

`RpnCalculator`

to convince yourself everything is connected properly.```
Feature: Basic Arithmetic# Try to change one of the values below to provoke a failure
Scenario: Addition
Given a calculator I just turned on
When I push 4
And I push 5
And I add
Then the result is 9# Try to change one of the values below to provoke a failure
Scenario: Another Addition
Given a calculator I just turned on
When I push 4
And I push -7
And I add
Then the result is -3Scenario Outline: Subtract
Given a calculator I just turned on
When I push <a>
And I push <b>
And I subtract
Then the result is <result> Examples:
| a | b | result |
| 3 | 2 | 1 |
| 2 | 3 | -1 |
```

Implement the steps and make the scenarios pass.