input: def closest_vowel(word):
      """Given a string, determine the nearest vowel positioned between 
      two consonants from the last vowel in the word (case sensitive).
      
      Vowels at the start or end of the word should not be considered. Return an empty string if 
      no vowels meet this criterion. 
  
      Assume the input string consists solely of English letters.
  
      Example:
      closest_vowel("yogurt") ==> "u"
      closest_vowel("FULL") ==> "U"
      closest_vowel("quick") ==> ""
      closest_vowel("ab") ==> ""
      """
output:     
      vowels = 'aeiouAEIOU'
      closest = ''
  
      for i in range(len(word)-2, 0, -1):
          if word[i] in vowels:
              if word[i-1] not in vowels and word[i+1] not in vowels:
                  closest = word[i]
                  break
                  
      return closest
  