import collections

def flatten1(seq):
    """Returns a list containing all the items in seq with no nested
    sequences

    >>> a = [[1, 2], [3, [4, [5, 6], 7], 8], 9, [10, 11], [12]]
    >>> b =flatten1(a)
    >>> b
    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
    >>> flatten1(b) == b
    True
    >>> c = [["S"] * 4, ["M"] * 2, "R", "H", ["D"] * 3, "A", "B", "E"]
    >>> flatten1(c)
    ['S', 'S', 'S', 'S', 'M', 'M', 'R', 'H', 'D', 'D', 'D', 'A', 'B', 'E']
    """
    result = []
    for x in seq:
        if isinstance(x, list):
            result.extend(flatten1(x))
        else:
            result.append(x)
    return result


def flatten2(seq):
    """Returns a list containing all the items in seq with no nested
    sequences

    >>> a = [[1, 2], [3, [4, [5, 6], 7], 8], 9, [10, 11], [12]]
    >>> b =flatten2(a)
    >>> b
    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
    >>> flatten2(b) == b
    True
    >>> c = [["S"] * 4, ["M"] * 2, "R", "H", ["D"] * 3, "A", "B", "E"]
    >>> flatten2(c)
    ['S', 'S', 'S', 'S', 'M', 'M', 'R', 'H', 'D', 'D', 'D', 'A', 'B', 'E']
    """
    result = []
    for x in seq:
        if isinstance(x, collections.Sequence):
            result.extend(flatten2(x))
        else:
            result.append(x)
    return result


if __name__ == "__main__":
    import doctest
    doctest.testmod()