def distinct_letters(X):
    """
    Diagnostic function
    :param X: a data matrix: a list wrapping a list of strings, with each sublist being a sentence.
    :return:
    >>> dl = distinct_letters([['the', 'quick', 'brown'],['how', 'now', 'cow']])
    >>> sorted(dl)
    ['b', 'c', 'e', 'h', 'i', 'k', 'n', 'o', 'q', 'r', 't', 'u', 'w']
    """
    return set([letter
                for sentence in X
                for word in sentence
                for letter in word])