def print_matrix(matrix, msg=None):
    """
    Convenience function: Displays the contents of a matrix of integers.

    :Parameters:
        matrix : list of lists
            Matrix to print

        msg : str
            Optional message to print before displaying the matrix
    """
    import math

    if msg is not None:
        print(msg)

    # Calculate the appropriate format width.
    width = 0
    for row in matrix:
        for val in row:
            width = max(width, int(math.log10(val)) + 1)

    # Make the format string
    format = '%%%dd' % width

    # Print the matrix
    for row in matrix:
        sep = '['
        for val in row:
            sys.stdout.write(sep + format % val)
            sep = ', '
        sys.stdout.write(']\n')