input: def count_hexadecimal_primes(hex_str):
      """Write a function that receives a hexadecimal number as a string 
      and calculates how many of the digits in the string are prime numbers 
      (in base 10, prime numbers are numbers greater than 1 that are not 
      divisible by any number other than 1 and themselves).
      Hexadecimal digits include: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F.
      Relevant prime numbers to look for are: 2, 3, 5, 7, B (=decimal 11), 
      and D (=decimal 13).
      Assume the input is either correct or an empty string, and hexadecimal 
      characters A, B, C, D, E, F are in uppercase.
      Examples:
      For hex_str = "AB", the output should be 1.
      For hex_str = "1077E", the output should be 2.
      For hex_str = "ABED1A33", the output should be 4.
      For hex_str = "123456789ABCDEF0", the output should be 6.
      For hex_str = "2020", the output should be 2.
      """
output:     prime_digits = {'2', '3', '5', '7', 'B', 'D'}
      count = 0
      for char in hex_str:
          if char in prime_digits:
              count += 1
      return count