The children of a group are ordered with respect to each other. This method makes the newobj be in the group at the position before the child, so that the newobj will be painted immediately before the child. If the newobj already belonged to this group, only its position in the list is changed. The newobj must not already be part of a different group or be a top-level object in some layer. The existing child must be either null or a member of this group (but not be the same as newobj). If the child is null, the newobj is added at the beginning of the list, behind all other objects in this group.