def find_result_sentiment(words, word_sentiments):
    """Compute average sentiment for a result represented as a list of words.
        Words not in word_sentiments are ignored.
        If we don't have sentiment for any wd in words, return None.
    """
    word_ratings = [word_sentiments[wd] for wd in words 
        if wd in word_sentiments]
    
    if len(word_ratings) == 0:
        return None

    return sum(word_ratings) / len(word_ratings)