diff --git a/libethereum/Common.h b/libethereum/Common.h
index ec69288ee..01918dea0 100644
--- a/libethereum/Common.h
+++ b/libethereum/Common.h
@@ -65,7 +65,7 @@ class FixedHash
 	using Arith = boost::multiprecision::number<boost::multiprecision::cpp_int_backend<_N * 8, _N * 8, boost::multiprecision::unsigned_magnitude, boost::multiprecision::unchecked, void>>;
 
 public:
-	static const unsigned size = _N;
+	enum { size = _N };
 
 	FixedHash() { m_data.fill(0); }
 	FixedHash(Arith const& _arith) { toBigEndian(_arith, m_data); }
diff --git a/libethereum/State.h b/libethereum/State.h
index 4ea93c1a4..9ed967c4e 100644
--- a/libethereum/State.h
+++ b/libethereum/State.h
@@ -154,8 +154,11 @@ private:
 	/// Sets m_currentBlock to a clean state, (i.e. no change from m_previousBlock).
 	void resetCurrent();
 
-	void mergeOverlay() { for (auto const& i: m_over) m_db->Put(m_writeOptions, ldb::Slice((char const*)i.first.data(), i.first.size), ldb::Slice(i.second.data(), i.second.size())); m_over.clear(); }
-	void dropOverlay() { m_over.clear(); }
+	/// Commit all pending state modifications for archival. This cannot be undone.
+	void commit() { for (auto const& i: m_over) m_db->Put(m_writeOptions, ldb::Slice((char const*)i.first.data(), i.first.size), ldb::Slice(i.second.data(), i.second.size())); m_over.clear(); }
+
+	/// Rollback all pending state modifictions.
+	void rollback() { m_over.clear(); }
 
 	ldb::DB* m_db;								///< The DB, holding all of our Tries' backend data.
 	ldb::WriteOptions m_writeOptions;
diff --git a/libethereum/Trie.h b/libethereum/Trie.h
index b03bca15c..fa632ac7f 100644
--- a/libethereum/Trie.h
+++ b/libethereum/Trie.h
@@ -86,6 +86,8 @@ public:
 
 private:
 	std::string node(h256 _h) const { if (_h == c_null) return std::string(); if (m_over) { auto it = m_over->find(_h); if (it != m_over->end()) return it->second; } std::string ret; m_db->Get(m_readOptions, ldb::Slice((char const*)&m_root, 32), &ret); return ret; }
+	void insertNode(h256 _h, bytesConstRef _v) const {}
+	void killNode(h256 _h) const {}	// only from overlay - no killing from DB proper.
 
 	static const h256 c_null;
 	h256 m_root = c_null;
diff --git a/libethereum/vector_ref.h b/libethereum/vector_ref.h
index e68dac999..facc5621e 100644
--- a/libethereum/vector_ref.h
+++ b/libethereum/vector_ref.h
@@ -25,6 +25,7 @@ public:
 
 	explicit operator bool() const { return m_data && m_count; }
 
+	bool contentsEqual(std::vector<_T> const& _c) const { return _c.size() == m_count && !memcmp(_c.data(), m_data, m_count); }
 	std::vector<_T> toVector() const { return std::vector<_T>(m_data, m_data + m_count); }
 	std::string toString() const { return std::string((char const*)m_data, ((char const*)m_data) + m_count); }
 	template <class _T2> operator vector_ref<_T2>() const { assert(m_count * sizeof(_T) / sizeof(_T2) * sizeof(_T2) / sizeof(_T) == m_count); return vector_ref<_T2>((_T2*)m_data, m_count * sizeof(_T) / sizeof(_T2)); }