induction b with d hd rw [add_zero, zero_add] rfl rw [add_succ, succ_add] rw [hd] rfl i really enjoyed finally internal

induction b with d hd rw [add_zero, zero_add] rfl rw [add_succ, succ_add] rw [hd] rfl i really enjoyed finally internalizing dependent type theory. it helped a lot with that.