input: def count_vowels_on_even_indices(s):
      """
      Given a string s, calculate the number of uppercase vowels located at even positions.
  
      For example:
      count_vowels_on_even_indices('aBCdEf') returns 1
      count_vowels_on_even_indices('abcdefg') returns 0
      count_vowels_on_even_indices('dBBE') returns 0
      """
output:     
      vowels = set("AEIOU")
      count = 0
      for i in range(0, len(s), 2):
          if s[i] in vowels:
              count += 1
      return count
  