diff --git a/.local/share/common-lisp/source/ambrevar/shell.lisp b/.local/share/common-lisp/source/ambrevar/shell.lisp
index bdb37af7..dfece85f 100644
--- a/.local/share/common-lisp/source/ambrevar/shell.lisp
+++ b/.local/share/common-lisp/source/ambrevar/shell.lisp
@@ -399,3 +399,13 @@ INTERFACE is a string in the form of `wlp2s0'."
               "sudo sh -c 'echo 1 > /sys/module/snd_hda_intel/parameters/power_save'")))
     (enable-audio-codec-power-management)
     (bbswitch-off)))
+
+(export-always 'find-missing-files)
+(defun find-missing-files (source target)
+  "Report files from SOURCE that are not in TARGET.
+Files are compared by size."
+  (let ((source-files (uiop:with-current-directory (source)
+                        (fof:finder)))
+        (target-files (uiop:with-current-directory (target)
+                        (fof:finder))))
+    (set-difference source-files target-files :key #'fof:size)))