@pimms.value<EOL><INDENT>def white_indices(lh_white_indices, rh_white_indices):<DEDENT>
if   lh_white_indices is None and rh_white_indices is None: return None<EOL>elif lh_white_indices is None: return rh_white_indices<EOL>elif rh_white_indices is None: return lh_white_indices<EOL>else: return frozenset(lh_white_indices | rh_white_indices)<EOL>
sub.white_indices is a frozenset of the indices of the white voxels in the given subject
represented as 3-tuples.