def get_letters_seq(data):
    """
        Get letters list and letter-index dictionaries
    """
    # Letters of all words
    a1 = [set(x) for l in data.values() for x in l]
    # All letters (20)
    letter = ['<PAD>'] + list(set([x for l in a1 for x in l]))
    # Letters-index dictionaries
    letter_to_ix = dict((i, j) for j, i in enumerate(letter))
    ix_to_letter = dict((j, i) for j, i in enumerate(letter))
    return letter, letter_to_ix, ix_to_letter