--- a/reference/quadrilaterals.ldr	Tue Jan 23 14:09:52 2018 +0200
+++ b/reference/quadrilaterals.ldr	Tue Jan 23 15:30:48 2018 +0200
@@ -17,3 +17,7 @@
 4 16 -5 0 40 -15 0 35 -5 0 30 5 0 35
 0 // collinear
 4 16 20 0 40 30 0 45 30 0 50 30 0 55
+0 // crossed and collinear
+4 16 -50 0 10 -40 0 15 -40 0 10 -40 0 20
+0 // concave and skew
+4 16 -30 0 -20 -35 5 -5 -30 0 -10 -25 0 -5