diff --git a/slicer/slice.c b/slicer/slice.c
index 38b794b..cdc05aa 100644
--- a/slicer/slice.c
+++ b/slicer/slice.c
@@ -71,17 +71,26 @@ static void mark_z(float z)
 
 static void add(const struct v *a, const struct v *b, const struct v *c)
 {
+	float cross;
 	struct z *z;
 	struct line *line;
 
-	/* @@@ should check that C is above AB */
-
 	if (eq(a->x, b->x) && eq(a->y, b->y)) {
 fprintf(stderr, "zero point\n");
 		return;
 	}
+
 	if (c->z < a->z)
 		return;
+
+	cross = (b->x - a->x) * (c->y - a->y) - (b->y - a->y) * (c->x - a->x);
+	if (!eq(cross, 0)) {
+		fprintf(stderr,
+		    "inclined facet\n\t%f %f %f\n\t%f %f %f\n\t%f %f %f\n",
+		    a->x, a->y, a->z, b->x, b->y, b->z, c->x, c->y, c->z);
+		exit(1);
+	}
+
 	z = get_z(a->z);
 	line = alloc_type(struct line);
 	line->ax = a->x;