diff -r 12d4ddc4bfd8 -r d383f319f35b tests/concave.py
--- a/tests/concave.py	Sun Jan 21 15:03:38 2018 +0200
+++ b/tests/concave.py	Mon Jan 22 12:22:01 2018 +0200
@@ -6,57 +6,31 @@
     # Returns whether all elements in container have the same sign
     return min(container) * max(container) >= 0
 
-def transform_to_xy(geometry):
-    a, b, c = geometry.vertices[:3]
-    
-
 def concave_test(model):
     for quadrilateral in model.quadrilaterals:
-        print([cross_product(v2 - v1, v3 - v1) for v1, v2, v3 in pairs(quadrilateral.geometry.vertices, count = 3)])
+        geometry = transform_to_xy(quadrilateral.geometry)
         z_scores = [
             cross_product(v2 - v1, v3 - v1).z
-            for v1, v2, v3 in pairs(quadrilateral.geometry.vertices, count = 3)
+            for v1, v2, v3 in pairs(geometry.vertices, count = 3)
         ]
-        print(z_scores)
         if not sign_consistency(z_scores):
-            yield warning(quadrilateral, 'Concave quadrilateral')
+            yield error(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)
+        for triangles in split_quadrilateral(quadrilateral.geometry):
+            plane_1 = triangle_plane_normal(triangles[0])
+            plane_2 = triangle_plane_normal(triangles[1])
             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
+
+manifest = {
+    'tests': {skew_test, concave_test},
+}