input: Write a function to determine if a number provided by the user is prime or not. The function should return a string indicating whether the number is prime or not. If the number is 1, it should return "1 is not a prime number."
  
  
  def is_prime(num):
   if num == 1:
   return "1 is not a prime number."
   is_prime = True
   for i in range(2, num):
   if num % i == 0:
   is_prime = False
   break
   if is_prime:
   return f"{num} is a prime number."
   else:
   return f"{num} is not a prime number."
  
  # Test cases
  assert is_prime(17) == "17 is a prime number."
  assert is_prime(4) == "4 is not a prime number."
  assert is_prime(1) == "1 is not a prime number."
  assert is_prime(2) == "2 is a prime number."
  
output: def is_prime(num):
   if num == 1:
   return "1 is not a prime number."
   is_prime = True
   for i in range(2, num):
   if num % i == 0:
   is_prime = False
   break
   if is_prime:
   return f"{num} is a prime number."
   else:
   return f"{num} is not a prime number."