def isPalindrome(s):
    """ Assumes s is a str
        Returns True if the 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))
