theorem elb10 (b: nat): $ 0 e. b1 b $;
0 e. b1 b <-> 0 = 0 \/ 0 - 1 e. b
0 = 0 -> 0 = 0 \/ 0 - 1 e. b
0 = 0
0 = 0 \/ 0 - 1 e. b
0 e. b1 b