Fixpoint power (base : nat) (exp : nat) : nat :=
 match exp with
 | 0 => 1
 | S n => base * (power base n)
 end.

Lemma countBLeafCT_join_eq: forall n s1 s2 s3, join s1 s2 s3 -> height s1 <= n -> height s2 <= n -> countBLeafCT n s1 + countBLeafCT n s2 = countBLeafCT n s3.