def a
  puts 'a'
  'a'
end

def null
  puts 'null'
  nil
end

def memoize_1
  @@memoize_1 ||= null or a
end

def memoize_2
  @@memoize_2 ||= null || a
end

puts "== memoize_1"
3.times do
  memoize_1
end

puts "== memoize_2"
3.times do
  memoize_2
end

# == memoize_1
# null
# a
# null
# a
# null
# a
# == memoize_2
# null
# a