5 files changed, 115 insertions, 4 deletions

diff --git a/starts/meaning-vm/level-0/baseref.hpp b/starts/meaning-vm/level-0/baseref.hpp
index 9244492..b6e98e8 100644
--- a/starts/meaning-vm/level-0/baseref.hpp
+++ b/starts/meaning-vm/level-0/baseref.hpp
@@ -13,8 +13,8 @@ namespace level0 {
 template <typename ref>
 class baseref
 {
-	struct array; struct links_t;
 public:
+	struct array; struct links_t;
 	baseref(concept *p)
 	: p(p)
 	{
@@ -63,6 +63,9 @@ public:
 	T& val() { return p->val<T>(); }
 	template <typename T>
 	void val(T const & v) { p->val<T>(v); }
+	bool hasval() { return p->hasval(); }
+	template <typename T>
+	bool hasvalof() { return p->hasvalof<T>(); }
 
 	operator concept*() const { return p; }
 	concept*& ptr() { return p; }
@@ -91,9 +94,15 @@ public:
 	operator level9::ref &() { return *reinterpret_cast<level9::ref*>(this); }
 
 	bool operator==(ref const & other) const { return self.p == other.p; }
-	bool operator!=(ref const & other) const { return self.p == other.p; }
+	bool operator!=(ref const & other) const { return self.p != other.p; }
 	bool operator<(ref const & other) const { return self.p < other.p; }
 
+	bool crucial() { return self.p->crucial(); }
+	bool crucial(ref type, ref target) { return self.p->crucial(type.p, target.p); }
+
+	void setcrucial() { self.p->setcrucial(); }
+	void setcrucial(ref type, ref target) { self.p->setcrucial(type.p, target.p); }
+
 protected:
 	concept * p;
 
@@ -107,16 +116,21 @@ private:
 
 		mutit & operator++() { ++ self.it; return self; }
 		mutit operator++(int i) { return self.it.operator++(i); }
+		mutit & operator--() { -- self.it; return self; }
+		mutit operator--(int i) { return self.it.operator--(i); }
 		bool operator==(mutit const & other) const { return self.it == other.it; }
 		bool operator!=(mutit const & other) const { return self.it != other.it; }
 
 		val & operator*() { return *(val*)&self.it.operator*(); }
 		val * operator->() { return (val*)self.it.operator->(); }
 
+		It & underlying() { return it; }
+
 	private:
 		It it;
 	};
 
+public:
 	struct array
 	{
 		using iterator = mutated_it<ref,typename concept::array::iterator>;
@@ -134,6 +148,10 @@ private:
 
 		decltype(concept::links) & links;
 	};
+
+	void unlink(typename links_t::iterator & it) { p->unlink(it.underlying()); }
+	bool crucial(typename links_t::iterator it) { return self.p->crucial(it.underlying()); }
+	void setcrucial(typename links_t::iterator it) { self.p->setcrucial(it.underlying()); }
 };
 
 template <typename ref>
diff --git a/starts/meaning-vm/level-0/concept.cpp b/starts/meaning-vm/level-0/concept.cpp
index 0661edd..373b3b7 100644
--- a/starts/meaning-vm/level-0/concept.cpp
+++ b/starts/meaning-vm/level-0/concept.cpp
@@ -16,16 +16,49 @@ void concept::link(concept* type, concept* target)
 	links.insert({type, target});
 }
 
+bool concept::crucial(concept* type, concept* target)
+{
+	auto ls = links.equal_range(type);
+	bool wascrucial = false;
+	bool wasnotcrucial = false;
+	for (auto l = ls.first; l != ls.second; ++ l) {
+		if (l->second == target) {
+			if (crucialparts.count(l)) { wascrucial = true; }
+			else { wasnotcrucial = true; }
+		}
+	}
+	if (wascrucial && wasnotcrucial) { throw link_type_not_unique(selfref, type); }
+	if ((!wascrucial) && (!wasnotcrucial)) { throw no_such_link_type(selfref, type); }
+	return wascrucial;
+}
+
+void concept::setcrucial(concept* type, concept* target)
+{
+	auto ls = links.equal_range(type);
+	for (auto l = ls.first; l != ls.second; ++ l) {
+		if (l->second == target) {
+			if (!crucialparts.count(l)) {
+				setcrucial(l);
+				return;
+			}
+		}
+	}
+	throw no_such_link_type(selfref, type);
+}
+
 void concept::unlink(concept* type, concept* target)
 {
 	auto ls = links.equal_range(type);
+	bool wascrucial = false;
 	for (auto l = ls.first; l != ls.second; ++ l) {
 		if (l->second == target) {
+			if (crucialparts.count(l)) { wascrucial = true; continue; }
 			links.erase(l);
 			return;
 		}
 	}
-	throw no_such_link_type_target(selfref, type, target);
+	if (wascrucial) { throw crucial_link_type_target(selfref, type, target); }
+       	throw no_such_link_type_target(selfref, type, target);
 }
 
 void concept::unlink(concept* type)
@@ -39,7 +72,15 @@ void concept::unlink(concept* type)
 	if (mid != ls.second) {
 		throw link_type_not_unique(selfref, type);
 	}
-	links.erase(ls.first);
+	unlink(ls.first);
+}
+
+void concept::unlink(decltype(links)::iterator & it)
+{
+	if (crucialparts.count(it)) {
+		throw crucial_link_type_target(selfref, it->first, it->second);
+	}
+	links.erase(it++);
 }
 
 bool concept::linked(concept* type) const
diff --git a/starts/meaning-vm/level-0/concept.hpp b/starts/meaning-vm/level-0/concept.hpp
index d42d36b..e885ccf 100644
--- a/starts/meaning-vm/level-0/concept.hpp
+++ b/starts/meaning-vm/level-0/concept.hpp
@@ -4,6 +4,7 @@
 
 #include <any>
 #include <map>
+#include <unordered_set>
 #include <vector>
 
 namespace intellect {
@@ -23,6 +24,14 @@ struct concept
 	void link(concept* type, concept* target);
 	void unlink(concept* type, concept* target);
 	void unlink(concept* type);
+	void unlink(decltype(links)::iterator & it);
+
+	bool crucial() { return iscrucial || crucialparts.size(); }
+	bool crucial(concept* type, concept* target);
+	bool crucial(decltype(links)::iterator it) { return crucialparts.count(it); }
+	void setcrucial() { iscrucial = true; }
+	void setcrucial(concept* type, concept* target);
+	void setcrucial(decltype(links)::iterator it) { crucialparts.insert(it); }
 
 	bool linked(concept* type) const;
 	bool linked(concept* type, concept* target) const;
@@ -41,6 +50,23 @@ struct concept
 
 	template <typename T>
 	void val(T const & v) { data = v; }
+
+	bool hasval() { return data.has_value(); }
+
+	template <typename T>
+	bool hasvalof() { return hasval() && data.type() == typeid(T); }
+
+private:
+	// for permanence
+	bool iscrucial;
+	struct linksit_hash
+	{
+		size_t operator()(decltype(links)::iterator const &it) const
+		{
+			return std::hash<decltype(&*it)>()(&*it);
+		}
+	};
+	std::unordered_set<decltype(links)::iterator, linksit_hash> crucialparts;
 };
 
 }
diff --git a/starts/meaning-vm/level-0/errors.hpp b/starts/meaning-vm/level-0/errors.hpp
index d46eef7..e599261 100644
--- a/starts/meaning-vm/level-0/errors.hpp
+++ b/starts/meaning-vm/level-0/errors.hpp
@@ -32,6 +32,30 @@ struct no_such_link_type_target : public std::out_of_range
 	concept* const target;
 };
 
+struct crucial_link_type_target : public std::out_of_range
+{
+	crucial_link_type_target(concept* source, concept* type, concept* target)
+	: std::out_of_range("concept part is crucial"),
+	  source(source),
+	  type(type),
+	  target(type)
+	{ }
+
+	concept* const source;
+	concept* const type;
+	concept* const target;
+};
+
+struct crucial_concept : public std::invalid_argument
+{
+	crucial_concept(concept* topic)
+	: std::invalid_argument("concept is crucial"),
+	  topic(topic)
+	{ }
+
+	concept* const topic;
+};
+
 struct link_type_not_unique : public std::invalid_argument
 {
 	link_type_not_unique(concept* source, concept* type)
diff --git a/starts/meaning-vm/level-0/memorystore.cpp b/starts/meaning-vm/level-0/memorystore.cpp
index 04aadd5..1d9e9f6 100644
--- a/starts/meaning-vm/level-0/memorystore.cpp
+++ b/starts/meaning-vm/level-0/memorystore.cpp
@@ -89,6 +89,7 @@ static concept* referenced(ref r, concept* source = 0) {
 
 void basic_dealloc(ref r)
 {
+	if (r.crucial()) { throw crucial_concept(r); }
 	auto it = index().find(r);
 	if (it == index().end()) { throw no_such_concept(r); }
 
@@ -106,6 +107,7 @@ void dealloc_from(ref source)
 
 	auto ours = source.getAll(concepts::allocates());
 	for (auto allocation : ours) {
+		if (allocation.crucial()) { throw crucial_concept(allocation); }
 		source.unlink(concepts::allocates(), allocation);
 		allocation.unlink(concepts::allocator(), source);
 		if (allocation.linked(concepts::allocator())) { continue; }