diff -r eb74680a5e43 -r 12d4ddc4bfd8 tests/concave.py
--- a/tests/concave.py	Fri Jan 19 13:41:23 2018 +0200
+++ b/tests/concave.py	Sun Jan 21 15:03:38 2018 +0200
@@ -1,5 +1,5 @@
-from math import degrees, pi as π
-from testsuite import warning
+from math import acos, degrees, radians, pi as π
+from testsuite import warning, error
 from geometry import *
 
 def sign_consistency(container):
@@ -11,7 +11,7 @@
     
 
 def concave_test(model):
-    for quadrilateral in model:
+    for quadrilateral in model.quadrilaterals:
         print([cross_product(v2 - v1, v3 - v1) for v1, v2, v3 in pairs(quadrilateral.geometry.vertices, count = 3)])
         z_scores = [
             cross_product(v2 - v1, v3 - v1).z
@@ -19,4 +19,44 @@
         ]
         print(z_scores)
         if not sign_consistency(z_scores):
-            yield warning(quadrilateral, 'Concave quadrilateral')
\ No newline at end of file
+            yield warning(quadrilateral, 'Concave quadrilateral')
+
+def bowtie_quadrilateral_test(model):
+    for quadrilateral in model.quadrilaterals:
+        vertices = IndexRing(quadrilateral.geometry.vertices)
+        for i in (0, 1):
+            line1 = LineSegment(vertices[0 + i], vertices[1 + i])
+            line2 = LineSegment(vertices[2 + i], vertices[3 + i])
+            try:
+                line_intersection(line1, line2)
+            except NoIntersection:
+                pass
+            else:
+                yield error(quadrilateral, 'Bowtie quadrilateral')
+                break
+
+def vector_angle(vec_1, vec_2, normalized = False):
+    cosine = dot_product(vec_1, vec_2)
+    try:
+        cosine /= vec_1.length() * vec_2.length()
+    except ZeroDivisionError:
+        return 0
+    angle = acos(cosine)
+    if normalized and angle > π / 2:
+        angle = π - angle
+    return angle
+
+def skew_test(model):
+    for quadrilateral in model.quadrilaterals:
+        vertices = IndexRing(quadrilateral.geometry.vertices)
+        for i in (0, 1):
+            a, b = vertices[0 + i], vertices[1 + i]
+            c, d = vertices[2 + i], vertices[3 + i]
+            plane_1 = cross_product(b - a, d - a)
+            plane_2 = cross_product(d - c, b - c)
+            angle = vector_angle(plane_1, plane_2, normalized = True)
+            if angle > radians(0.1):
+                yield error(quadrilateral,
+                    'Skew quadrilateral (plane angle {}°)',
+                    '%.3f' % degrees(angle))
+                break