text: import math
  
  
  def prime(n: int) -> int:
      """Computes all prime numbers below n.
      Computes the sieve of Eratosthenes
      >>> prime(20)
      {1, 2, 3, 5, 7, 11, 13, 17, 19}
      """
      assert n > 0, "Positive integers only"
      p = set(range(1, n))
      for i in range(2, int(math.sqrt(n)) + 1):
          p = p - set(x*i for x in range(2, n//i + 1))
      return p
  
  
  prime(20)