def check_dense_hemi(hemisphere):
    """
    Check hemisphere argument for compatibility.

    Parameters
    ----------
    hemisphere : 'left' or 'right' or None
        if bilateral, use None

    Returns
    -------
    'left' or 'right' or None

    Raises
    ------
    ValueError : invalid hemisphere argument

    """
    options = ['left', 'l', 'L', 'right', 'r', 'R', None, 'lr', 'LR']
    if hemisphere not in options:
        raise ValueError('{} is not a valid hemisphere'.format(hemisphere))
    if hemisphere in ['left', 'l', 'L']:
        return 'left'
    if hemisphere in ['right', 'r', 'R']:
        return 'right'
    if hemisphere in ['None', 'lr', 'LR']:
        return None
