diff --git a/solidify/overlap.c b/solidify/overlap.c
index 572495c..a8c7805 100644
--- a/solidify/overlap.c
+++ b/solidify/overlap.c
@@ -245,7 +245,7 @@ static void draw_map(GtkWidget *widget, struct solid *s)
 		.b = { 0, 0 },
 	};
 	struct matrix mb = {
-		.a = { { 1, 0 }, { 0, 1 } },	/* @@@ why not a[1][1] = -1 ? */
+		.a = { { -1, 0 }, { 0, 1 } },
 		.b = { 0, 0 },
 	};