1.upto 15 do |i|
  (out ||= '') << 'Fizz' if (i % 3).zero?
  (out ||= '') << 'Buzz' if (i % 5).zero?
  puts out || i
  # This will print FizzBuzz as expected
end

1.upto 15 do |i|
  out ||= '' << 'Fizz' if (i % 3).zero?
  out ||= '' << 'Buzz' if (i % 5).zero?
  puts out || i
  # Omitting the parenthesis around the conditional assignment will fail printing FizzBuzz at 15
end