input: def special_letter_count(word: str) -> int:
      """
      Given a string word, a character is considered special if it appears in both its lowercase and uppercase forms within the string.
  
      Return the count of such special letters in word.
  
      Examples:
      >>> word = "aaAbcBC"
      3
      >>> word = "abc"
      0
      >>> word = "abBCab"
      1
  
      Constraints:
      1 <= word.length <= 50
      The string consists solely of English letters, both lowercase and uppercase.
      """
output:     
      special_letters = set()
      for char in word:
          if char.islower() and char.upper() in word:
              special_letters.add(char)
          elif char.isupper() and char.lower() in word:
              special_letters.add(char.lower())
      return len(special_letters)
  