def _set_center(el, pos):
    """Set element <el>'s center using its storer_rect as reference."""
    center_x, center_y = pos
    x_shift = 0
    y_shift = 0
    if center_x is not None:
        x_shift = center_x - el.get_storer_rect().centerx
    if center_y is not None:
        y_shift = center_y - el.get_storer_rect().centery
    el.move((x_shift, y_shift))
