d
      then the stack should be empty

    scenario stack with one value is not empty
      given an empty stack with one pushed value
      then the stack should not be empty

    scenario peek is called
      given a stack containing an item
      when peek is called
      then it should provide the value of the most recent pushed value
      then the stack should not be empty
      then calling pop should also return the peeked value which is \
        the same as the original pushed value
