Organize domain project.

Shogi.Domain/YetToBeAssimilatedIntoDDD/DomainExtensions.cs

@@ -0,0 +1,19 @@
+using Shogi.Domain.ValueObjects;
+
+namespace Shogi.Domain.YetToBeAssimilatedIntoDDD
+{
+    internal static class DomainExtensions
+    {
+        public static bool IsKing(this Piece self) => self.WhichPiece == WhichPiece.King;
+
+        public static bool IsBetween(this float self, float min, float max)
+        {
+            return self >= min && self <= max;
+        }
+
+        public static bool IsInsideBoardBoundary(this Vector2 self)
+        {
+            return self.X.IsBetween(0, 8) && self.Y.IsBetween(0, 8);
+        }
+    }
+}