Started to work on king threats

Sources/Engine/Extensions/Set+Union.swift

@@ -0,0 +1,9 @@
+extension Set {
+	static func + (lhs: Set<Element>, rhs: Set<Element>) -> Set<Element> {
+		return lhs.union(rhs)
+	}
+
+	static func += (lhs: inout Set<Element>, rhs: Set<Element>) {
+		lhs.formUnion(rhs)
+	}
+}