make nodes to provide nodes instead of mixins

include/links/OneToOneLink.h

@@ -5,10 +5,14 @@
 template <class TElem>
 class OneToOneLink : public BaseLink<TElem> {
 public:
-	OneToOneLink(std::shared_ptr<TElem> e) : BaseLink<TElem>(e) {}
-	void addChild(const std::shared_ptr<TElem>& child) override {
+	using ElemPtr = std::shared_ptr<TElem>;
+
+	OneToOneLink(ElemPtr e) : BaseLink<TElem>(e) {}
+
+	void addChild(const ElemPtr& child) override {
 		if (!this->children_.empty())
-			throw std::logic_error("OneToOneLink cannot have more than one child");
+			throw std::logic_error("Too many children");
+
 		BaseLink<TElem>::addChild(child);
 	}
 };