def _encode_structure_parts(local_target):
    """
    Encodes the design task, to distinguish between struture and wequence domains.

    Args:
        A sequence that may contain structure or sequence parts.

    Returns:
        A list representation of the sequence with all structure sites and None else.
    """
    encoding = [None] * len(local_target)
    for index, site in enumerate(local_target):
        if site in ['.', '(', ')', 'N']:
            encoding[index] = site
    return encoding