def find_sequence(a, b)
  stats = Hash.new(0)

  b.each { |el| stats[el] += 1 }
  a.select { |el| !prime?(stats[el]) }
end

def prime?(num)
  return false if num < 2

  (2..(num / 2)).each do |i|
    return false if num % i == 0
  end

  true
end