import itertools


def tupleofwords_to_wordoftuples(tupleofwords):
    """
    Transposes a tuple of words over the alphabet to a word of tuples.

    INPUT:

    - ``tupleofwords`` -- a tuple of a list of letters.

    OUTPUT:

    A list of tuples.

    Missing letters in the words are padded with the letter ``None``
    (from the empty word).

    EXAMPLES::

        sage: from sage.combinat.finite_state_machine import (
        ....:     tupleofwords_to_wordoftuples)
        sage: tupleofwords_to_wordoftuples(
        ....:     ([1, 2], [3, 4, 5, 6], [7]))
        [(1, 3, 7), (2, 4, None), (None, 5, None), (None, 6, None)]
    """
    return list(itertools.zip_longest(*tupleofwords, fillvalue=None))