add hierarchical node

include/links/OneToOneLink.h

@@ -0,0 +1,14 @@
+#pragma once
+#include "links/BaseLink.h"
+#include <stdexcept>
+
+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 {
+		if (!this->children_.empty())
+			throw std::logic_error("OneToOneLink cannot have more than one child");
+		BaseLink<TElem>::addChild(child);
+	}
+};