def classic_example_3(sequence_of_sequences,
                      what_to_count, what_to_mutate_into):
    """
    Shows counting and mutating in a sequence of LISTS.
      -- Counts and returns the number of 'what_to_count' occurrences.
      -- Mutates those occurrences into the 'what_to_mutate_into'.
    """
    print()
    print('------------------------------------------------')
    print('Classic example 3 on this sequence of LISTS:')
    print(sequence_of_sequences)
    print('------------------------------------------------')

    count = 0
    for k in range(len(sequence_of_sequences)):
        sequence = sequence_of_sequences[k]
        for j in range(len(sequence)):
            if sequence[j] == what_to_count:
                count = count + 1
                sequence[j] = what_to_mutate_into
    return count