diff --git a/diff.carp b/diff.carp
index 313388e..1996ff5 100644
--- a/diff.carp
+++ b/diff.carp
@@ -27,14 +27,10 @@
                                            (Array.unsafe-nth new inew)
                                            &[])]
           (for [i 0 (Array.length ol)]
-            (let-do [iold @(Array.unsafe-nth ol i)]
-              (set! _overlap
-                    (Map.put _overlap
-                             &iold
-                             &(Int.inc
-                               (if (/= iold 0)
-                                 (Map.get &overlap &(Int.dec iold))
-                                 iold))))
+            (let-do [iold @(Array.unsafe-nth ol i)
+                     new-val (Int.inc
+                       (if (/= iold 0) (Map.get &overlap &(Int.dec iold)) iold))]
+              (set! _overlap (Map.put _overlap &iold &new-val))
               (when-do (> (Map.get &_overlap &iold) sl)
                 (set! sl (Map.get &_overlap &iold))
                 (set! sso (Int.inc (- iold sl)))