def largest_number(seq_seq):
    """
    Returns the largest number in the subsequences of the given
    sequence of sequences.  Returns None if there are NO numbers
    in the subsequences.

    For example, if the given argument is:
        [(3, 1, 4),
         (13, 10, 11, 7, 10),
         [1, 2, 3, 4]]
    then this function returns 13.

    As another example, if the given argument is:
      ([], [-1111111111111111], [])
    then this function returns -1111111111111111.

    As yet another example, if the given argument is:
      ([], [], [])
    then this function returns None.

    Preconditions:
      :type seq_seq: (list, tuple)
    and the given argument is a sequence of sequences,
    where each subsequence contains only numbers.
    """
    # -------------------------------------------------------------------------
    # DONE: 3. Implement and test this function.
    #   Note that you should write its TEST function first (above).
    # -------------------------------------------------------------------------
    largest = 0
    for k in range(len(seq_seq)):
        for j in range(len(seq_seq[k])):
            if seq_seq[k][j] > largest or -seq_seq[k][j] > largest:
                largest = seq_seq[k][j]
    if largest != 0:
        return largest