Specs for calculator.
More tests for addition operation.
Specs for subtraction operation.
Update specs comments.
Remove extra spaces.
Introduce arithmetic module.
Add 'increment' private method.
Add ArithmeticSpy class to verify the 'increment' method correctness.
Improve specs context.
Add specs to capture behaviour when given nil operands.
Specs to capture behaviour when given single operand.
Rename spec context of subtraction operation. This fixes #7.
Replace 'before' with 'let'.
Extract spy injection and removal to into 'before(:all)' and after(:all)'.
Change arithmetic specs description and context to match module name and method.
Remove initial magic number of 'result'.
Spy should keep track of all previous operands. It should also clean up after each test.
Spy subtraction algorithm.
The 'spy_result' method will clear the result after each test.
Remove redundant calculator test fixture.