Index: src/Common/PersistentMap.h
===================================================================
--- src/Common/PersistentMap.h	(revision b419abbf67c78a1c8cb7d489fe599026745bad2b)
+++ src/Common/PersistentMap.h	(revision fdae91367ec3ec0ae0d11f084e14da4fd527b06c)
@@ -114,4 +114,11 @@
 	}
 
+	/// check if this is the only reference to itself
+	/// NOTE: optimizations employing this function are not thread-safe
+	bool is_shared(long local_ptrs = 0) const {
+		// 2 accounts for both the pointer which owns "this" and the one created for its use_count
+		return (this->shared_from_this().use_count() - local_ptrs) > 2;
+	}
+
 public:
 	using size_type = std::size_t;
@@ -144,6 +151,4 @@
 		// remove map from base
 		Base base_map = base->take_as<Base>();
-		base->reset_as_base();
-		// xxx -- investigate checking ref-count and omitting re-initialization if 1
 
 		// switch base to inverse of self and mutate base map
@@ -153,7 +158,10 @@
 				auto it = base_map.find( self.key );
 
-				base->init<Ins>( 
-					mut_this->shared_from_this(), std::move(self.key), std::move(it->second) );
-				base->mode = INS;
+				if ( base->is_shared( 1 ) ) {
+					base->reset_as_base();
+					base->init<Ins>( 
+						mut_this->shared_from_this(), std::move(self.key), std::move(it->second) );
+					base->mode = INS;
+				}
 
 				base_map.erase( it );
@@ -163,6 +171,9 @@
 				Ins& self = mut_this->as<Ins>();
 
-				base->init<Rem>( mut_this->shared_from_this(), self.key );
-				base->mode = REM;
+				if ( base->is_shared( 1 ) ) {
+					base->reset_as_base();
+					base->init<Rem>( mut_this->shared_from_this(), self.key );
+					base->mode = REM;
+				}
 
 				base_map.emplace( std::move(self.key), std::move(self.val) );
@@ -173,7 +184,10 @@
 				auto it = base_map.find( self.key );
 
-				base->init<Ins>( 
-					mut_this->shared_from_this(), std::move(self.key), std::move(it->second) );
-				base->mode = UPD;
+				if ( base->is_shared( 1 ) ) {
+					base->reset_as_base();
+					base->init<Ins>( 
+						mut_this->shared_from_this(), std::move(self.key), std::move(it->second) );
+					base->mode = UPD;
+				}
 
 				it->second = std::move(self.val);
@@ -240,5 +254,4 @@
 		// transfer map to new node
 		Ptr ret = std::make_shared<Self>( take_as<Base>() );
-		reset_as_base();
 		Base& base_map = ret->as<Base>();
 
@@ -246,13 +259,19 @@
 		auto it = base_map.find( k );
 		if ( it == base_map.end() ) {
-			// set self to REM node and insert into base
-			init<Rem>( ret, k );
-			mode = REM;
+			if ( is_shared() ) {
+				// set self to REM node and insert into base
+				reset_as_base();
+				init<Rem>( ret, k );
+				mode = REM;
+			}
 
 			base_map.emplace_hint( it, std::forward<K>(k), std::forward<V>(v) );
 		} else {
-			// set self to UPD node and modify base
-			init<Ins>( ret, std::forward<K>(k), std::move(it->second) );
-			mode = UPD;
+			if ( is_shared() ) {
+				// set self to UPD node and modify base
+				reset_as_base();
+				init<Ins>( ret, std::forward<K>(k), std::move(it->second) );
+				mode = UPD;
+			}
 
 			it->second = std::forward<V>(v);
@@ -271,10 +290,12 @@
 		// transfer map to new node
 		Ptr ret = std::make_shared<Self>( take_as<Base>() );
-		reset_as_base();
 		Base& base_map = ret->as<Base>();
 
-		// set self to INS node and remove from base
-		init<Ins>( ret, k, base_map[k] );
-		mode = INS;
+		if ( is_shared() ) {
+			// set self to INS node and remove from base
+			reset_as_base();
+			init<Ins>( ret, k, base_map[k] );
+			mode = INS;
+		}
 
 		base_map.erase( k );