Add forgotten fuzzy func

FuzzyMath/Functions/atan2.cs

@@ -0,0 +1,60 @@
+using System;
+
+namespace FuzzyMath
+{
+    public static partial class Functions
+    {
+
+        public static FuzzyNumber Atan2(FuzzyNumber Y, FuzzyNumber X)
+        {
+            var rotated = Y.Support.Contains(0) && !X.Support.Contains(0);
+
+            return FuzzyNumber.Map(Y, X, (y, x) => Atan2(y, x, rotated));
+        }
+
+        private static double Atan2Rotated(double y, double x)
+        {
+            if (x < 0 && y >= 0)
+            {
+                return Math.Atan(y / x) - Math.PI;
+            }
+
+            return Math.Atan2(y, x);
+        }
+
+        private static Interval Atan2(Interval y, Interval x, bool rotated)
+        {
+            double[] values;
+
+            if (rotated)
+            {
+                if (y.Contains(0) && x.Contains(0))
+                    return new Interval(-1.5 * Math.PI, 0.5 * Math.PI);
+
+                values = new double[4] {
+                    Atan2Rotated(y.A, x.A),
+                    Atan2Rotated(y.A, x.B),
+                    Atan2Rotated(y.B, x.A),
+                    Atan2Rotated(y.B, x.B)
+                };
+            }
+            else
+            {
+                if (y.Contains(0) && x.Contains(0))
+                    return new Interval(-Math.PI, Math.PI);
+
+                values = new double[4] {
+                    Math.Atan2(y.A, x.A),
+                    Math.Atan2(y.A, x.B),
+                    Math.Atan2(y.B, x.A),
+                    Math.Atan2(y.B, x.B)
+                };
+            }
+
+            Array.Sort(values);
+
+            return new Interval(values[0], values[3]);
+        }
+
+    }
+}