input: Write a function to determine if a given list of integers contains only prime numbers. If all elements in the list are prime numbers, return True; otherwise, return False. If the input list is empty, return True.
  assert is_all_prime([2, 3, 5, 7])==True
  assert is_all_prime([2, 4, 3, 5])==False
  assert is_all_prime([11, 13, 17, 19])==True
  assert is_all_prime([3, 5, 7, 9])==False
  assert is_all_prime([])==True
output: def is_all_prime(nums):
      if not nums:
          return True
      def is_prime(n):
          if n <= 1:
              return False
          for i in range(2, int(n**0.5) + 1):
              if n % i == 0:
                  return False
          return True
      for num in nums:
          if not is_prime(num):
              return False
      return True