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Chapter 5. Describing Code With RSpec
Recap:
  - jump from the outer circle(Cucumber) to the inner circle(RSpec) of the BDD cycle
  - TDD red/green/refactor cycle using RSpec
  - describe() method hooks into RSpec's API and returns a subclass of RSpec::Core::ExampleGroup
  - it() method creates an example
  - as_null_object()
  - let() takes a symbol representing a method name and block