Work on crab feet

src/Geometry/Vector.hs

@@ -67,6 +67,12 @@ angleVV a b
             d = a `dotV` b
          in acos $ d / (ma * mb)
 
+-- returns the angle opposite length a
+angleThreeSides :: Float -> Float -> Float -> Float
+angleThreeSides a b c = acos $ (f b + f c - f a) / (2*b*c)
+  where
+    f = (^ (2 ::Int))
+
 -- | Safe version of 'angleVV' that returns 0 if either vector is null.
 safeAngleVV :: Point2 -> Point2 -> Float
 {-# INLINE safeAngleVV #-}