Pick all that apply

• 1Choice 1.

  Verify that pop() and max return nil (or a defined error) on an empty stack

• 2Choice 2.

  Verify that after popping the current maximum, max goes back to the previous maximum

• 3Choice 3.

  Verify that pushing the same maximum twice and popping once keeps max

• 4Choice 4.

  Time a million push calls in a measure {} block and assert O(1) from the result

• 5Choice 5.

  Reach the internal auxiliary stack array with @testable import and check that the element layout matches what you expect