def write_output(entry, description, sequence):
    """Write a sequence and its header to its .txt file.

    While writing, also increase the marker counter and count the number of
    times this particular sequence has been found.

    :arg list entry: Library entry.
    :arg str description: Sequence header.
    :arg str sequence:  The sequence.
    """
    entry[3].write('>{}\n'.format(description))
    entry[3].write('{}\n'.format(sequence))
    entry[5] += 1
    if sequence in entry[6]:
        entry[6][sequence][1] += 1
    else:
        entry[6][sequence] = [description, 1]
