diff --git a/cameo/area.c b/cameo/area.c
index cf58116..364f4a5 100644
--- a/cameo/area.c
+++ b/cameo/area.c
@@ -84,6 +84,29 @@ static int is_inside(const struct path *a, const struct path *b)
 /*
  * Solve
  *
+ * ax+by = e
+ * cx+dy = f
+ *
+ * with Cramer's rule:
+ * http://en.wikipedia.org/wiki/Cramer's_rule
+ */
+
+static int cramer2(double a, double b, double c, double d, double e, double f,
+    double *x, double *y)
+{
+	double det;
+
+	det = a*d-b*c;
+	if (fabs(det) < EPSILON)
+		return 0;
+	*x = (e*d-b*f)/det;
+	*y = (a*f-e*c)/det;
+	return 1;
+}
+
+
+/*
+ * Solve
  * ax+na*bx = cx+nb*dx
  * ay+na*by = cy+nb*dy
  *
@@ -91,23 +114,12 @@ static int is_inside(const struct path *a, const struct path *b)
  *
  * na*bx + nb*-dx = cx-ax
  * na*by + nb*-dy = cy-ay
- *
- * which we the solve with Cramer's rule:
- * http://en.wikipedia.org/wiki/Cramer's_rule
  */
 
 static int intersect(double ax, double ay, double bx, double by,
     double cx, double cy, double dx, double dy, double *na, double *nb)
 {
-	double det;
-
-	det = dx*by-bx*dy;
-	if (fabs(det) < EPSILON)
-		return 0;
-
-	*na = (dx*(cy-ay)-dy*(cx-ax))/det;
-	*nb = (bx*(cy-ay)-by*(cx-ax))/det;
-	return 1;
+	return cramer2(bx, -dx, by, -dy, cx-ax, cy-ay, na, nb);
 }