g_return_val_if_fail (GTK_IS_CONTAINER (container), NULL);
g_return_val_if_fail (GTK_IS_WIDGET (child), NULL);
g_return_val_if_fail (child->parent == GTK_WIDGET (container), NULL);
g_return_val_if_fail (GTK_IS_CONTAINER (container), NULL);
g_return_val_if_fail (GTK_IS_WIDGET (child), NULL);
g_return_val_if_fail (child->parent == GTK_WIDGET (container), NULL);