diff --git a/src/Mono.cs b/src/Mono.cs
index 9d31865..6afa12c 100644
--- a/src/Mono.cs
+++ b/src/Mono.cs
@@ -298,4 +298,14 @@ static class Mono
         new XYZi(+1, +1, -1),
         new XYZi(+1, +1, +1)
     };
+
+    static XYZi InsetCell(XYZi cell)
+    {
+        // [!] don't inset past 0
+        return new(
+            cell.x - Maths.sign(cell.x),
+            cell.y - Maths.sign(cell.y),
+            cell.z - Maths.sign(cell.z)
+        );
+    }
 }