--- a/src/HOL/Data_Structures/Array_Braun.thy Thu Mar 21 15:51:04 2019 +0100
+++ b/src/HOL/Data_Structures/Array_Braun.thy Thu Mar 21 19:46:12 2019 +0100
@@ -279,7 +279,7 @@
"size_fast Leaf = 0" |
"size_fast (Node l x r) = (let n = size_fast r in 1 + 2*n + diff l n)"
-lemma diff: "braun t \<Longrightarrow> size t : {n, n + 1} \<Longrightarrow> diff t n = (if size t = n then 0 else 1)"
+lemma diff: "braun t \<Longrightarrow> size t : {n, n + 1} \<Longrightarrow> diff t n = size t - n"
by(induction t arbitrary: n) auto
lemma size_fast: "braun t \<Longrightarrow> size_fast t = size t"