fib.rb

def fib(x, sequence = []) 
  if x <= 1
    result = 1 
  elsif
    result1 = sequence[x - 1] ||= fib(x - 1)
    result2 = sequence[x - 2] ||= fib(x - 2)
    result = result1 + result2
  end
  sequence[x] = result
  return result
end

a = []
fib(a)