def isPalindrome(s):
   """Assumes s is a str
      Returns True if letters in s form a palindrome; False
        otherwise. Non-letters and capitalization are ignored."""
   
   def toChars(s):
      s = s.lower()
      letters = ''
      for c in s:
        if c in 'abcdefghijklmnopqrstuvwxyz':
            letters = letters + c
      return letters

   def isPal(s):
      if len(s) <= 1:
        return True
      else:
        return s[0] == s[-1] and isPal(s[1:-1])
         
   return isPal(toChars(s))