def get_participant_code(tier_marker_seq):
    """Return the participant code from a tier marker set.

    Parameters
    ----------
    tier_marker_seq : iterable of str
        A sequence of tier markers like ``{'CHI', '%mor', '%gra'}``

    Returns
    -------
    str
        A participant code, e.g., ``'CHI'``.
        Return ``None`` if no participant code is found.
    """
    for tier_marker in tier_marker_seq:
        if not tier_marker.startswith('%'):
            return tier_marker
    return None