Pad sentence sequences with EOL markers.

def mark_sentence_boundaries(sequences, drop=0.0):  # pragma: no cover
    """Pad sentence sequences with EOL markers."""
    for sequence in sequences:
        sequence.insert(0, "-EOL-")
        sequence.insert(0, "-EOL-")
        sequence.append("-EOL-")
        sequence.append("-EOL-")
    return sequences, None