input: def count_unique_case_letters(word: str) -> int:
      """
      Given a string word, identify letters that occur in both their lowercase and uppercase forms.
  
      Return the count of such unique letters in word.
  
      Examples:
      >>> word = "aaAbcBC"
      3
      >>> word = "abc"
      0
      >>> word = "abBCab"
      1
  
      Constraints:
      1 <= word.length <= 50
      word includes only English letters in both cases.
      """
output:     unique_letters = set()
      for char in word:
          if char.islower() and char.upper() in word:
              unique_letters.add(char.lower())
          elif char.isupper() and char.lower() in word:
              unique_letters.add(char.lower())
      return len(unique_letters)