File "stdin", line 32, characters 0-11:
Error:
In environment
T : Type
p : T -> bool
a : T
t1, t2 : btree T
IH1 : count p (rev_tree t1) = count p t1
IH2 : count p (rev_tree t2) = count p t2
Unable to unify "(if p a then 1 else 0) + (count p t1 + count p t2)" with
 "(if p a then 1 else 0) + (count p t2 + count p t1)".