          print('Test: Pop on non-empty stack')
          assert_equal(stack.pop(), 0)
          assert_equal(stack.min(), 1)
          assert_equal(stack.pop(), 3)
          assert_equal(stack.min(), 1)
          assert_equal(stack.pop(), 1)
          assert_equal(stack.min(), 5)
          assert_equal(stack.pop(), 5)
          assert_equal(stack.min(), sys.maxsize)
  
 
</s> remove         assert_equal(stack.min(), 5)
 </s> add         assert_equal(stack.minimum(), 5) </s> remove         assert_equal(stack.min(), 1)
 </s> add         assert_equal(stack.minimum(), 1) </s> remove         assert_equal(stack.min(), sys.maxsize)
 </s> add         assert_equal(stack.minimum(), sys.maxsize) </s> remove         assert_equal(stack.min(), 1)
 </s> add         assert_equal(stack.minimum(), 1) </s> remove         assert_equal(stack.min(), 1)
 </s> add         assert_equal(stack.minimum(), 1)
        assert_equal(stack.minimum(), 1)
          print('Test: Pop on non-empty stack')
          assert_equal(stack.pop(), 0)
          assert_equal(stack.min(), 1)
          assert_equal(stack.pop(), 3)
        assert_equal(stack.minimum(), 1)
          assert_equal(stack.pop(), 1)
          assert_equal(stack.min(), 5)
          assert_equal(stack.pop(), 5)
          assert_equal(stack.min(), sys.maxsize)
  
keep keep keep keep replace keep keep keep keep keep
 Update stack min challenge (#128)

Rename variable min_vals to stack_of_mins to improve clarity.
Rename method min to minimum to avoid using the same name as the built-in min. 
stacks_queues/stack_min/test_stack_min.py