(* LR *) | Node (B, x, Node (R, lx, ll , Node (R, lrx, lrl, lrr)), r ) -> Node (R, lrx, Node (B, lx , ll , lrl), Node (B, x , lrr, r ))
(* RL *) | Node (B, x, l , Node (R, rx, Node (R, rlx, rll, rlr), rr )) -> Node (R, rlx, Node (B, x , l , rll), Node (B, rx , rlr, rr ))
(* RR *) | Node (B, x, l , Node (R, rx, rl , Node (R, rrx, rrl, rrr))) -> Node (R, rx , Node (B, x , l , rl ), Node (B, rrx, rrl, rrr))
(* LR *) | Node (B, x, Node (R, lx, ll , Node (R, lrx, lrl, lrr)), r ) -> Node (R, lrx, Node (B, lx , ll , lrl), Node (B, x , lrr, r ))
(* RL *) | Node (B, x, l , Node (R, rx, Node (R, rlx, rll, rlr), rr )) -> Node (R, rlx, Node (B, x , l , rll), Node (B, rx , rlr, rr ))
(* RR *) | Node (B, x, l , Node (R, rx, rl , Node (R, rrx, rrl, rrr))) -> Node (R, rx , Node (B, x , l , rl ), Node (B, rrx, rrl, rrr))