s = Stack.new
s.empty? # => true
s.pop # => nil
s.pop(1) # => []
s.push(1) # => Stack[1]
s.push(2, 3) # => Stack[1, 2, 3]
s.pop # => 3
s.pop(1) # => [2]
s.empty? # => false
s = Stack[:a, :b, :c]
s.pop # => :c