def gap(g, inf, sup, gen = nil)
  gen = prime_range(inf, sup) if gen.nil?
  recent = gen.next
  current = gen.next
  return [recent, current] if current - recent == g
  gap(g, inf, sup, gen)
end

def prime_range(inf, sup)
  Enumerator.new do |enum|
    while inf < sup
      enum.yield inf if Prime.prime? inf
      inf += 1
    end
  end
end