diff --git a/README.md b/README.md
index a541924..8ccdfda 100644
--- a/README.md
+++ b/README.md
@@ -62,3 +62,6 @@ returns:
  {old={4, 5}, old_first=4, old_last=5, new={4, 5}, new_first=3, new_last=4},
  {old={},     old_first=6, old_last=5, new={6},    new_first=5, new_last=5}}
 ```
+
+(when diffing tables, it is guaranteed that for matching chunks, .old and .new refer to
+the same table, so `chunk.old == chunk.new` will succeed)