input: Write a function to generate a prime number sequence up to a given number n. If the number is less than 2, the function should return an empty list.
  
  assert generate_prime_numbers(10)==[2, 3, 5, 7]
  assert generate_prime_numbers(20)==[2, 3, 5, 7, 11, 13, 17, 19]
  assert generate_prime_numbers(1)==[]
  assert generate_prime_numbers(-5)==[]
output: def generate_prime_numbers(n):
      if n < 2:
          return []
      primes = []
      for num in range(2, n + 1):
          for i in range(2, int(num**0.5) + 1):
              if num % i == 0:
                  break
          else:
              primes.append(num)
      return primes