input: def count_upper_vowels_at_even_indices(s):
      """
      Given a string s, determine how many capital vowels appear at even positions.
  
      For example:
      count_upper_vowels_at_even_indices('aBCdEf') returns 1
      count_upper_vowels_at_even_indices('abcdefg') returns 0
      count_upper_vowels_at_even_indices('dBBE') returns 0
      """
output:     count = 0
      upper_vowels = {'A', 'E', 'I', 'O', 'U'}
      
      for i in range(0, len(s), 2):
          if s[i] in upper_vowels:
              count += 1
              
      return count