def find_faces_at_index(layer_qs, coord_val, index):
    """Find all the faces that intersect `coord_val` along `index`

    Example
    Slice a list of faces along the x-axis so that you can tell
    where to fill the shape in, by drawing lines across the polygon.
    """
    return [face for face_q in layer_qs for face in face_q
        if ((face.contour_points[0][index] > coord_val and face.contour_points[1][index] <= coord_val) or
            (face.contour_points[1][index] > coord_val and face.contour_points[0][index] <= coord_val))]