diff --git a/Ledger.cpp b/Ledger.cpp
index f1eba373a..d09612f17 100644
--- a/Ledger.cpp
+++ b/Ledger.cpp
@@ -433,3 +433,73 @@ void Ledger::addJson(Json::Value& ret)
 	else ledger["Closed"]=false;
 	ret[boost::lexical_cast<std::string>(mLedgerSeq)]=ledger;
 }
+
+Ledger::pointer Ledger::switchPreviousLedger(Ledger::pointer oldPrevious, Ledger::pointer newPrevious, int limit)
+{
+	// Build a new ledger that can replace this ledger as the active ledger,
+	// with a different previous ledger. We assume our ledger is trusted, as is its
+	// previous ledger. We make no assumptions about the new previous ledger.
+
+	int count;
+
+	// 1) Validate sequences and make sure the specified ledger is a valid prior ledger
+	if(newPrevious->getLedgerSeq()!=oldPrevious->getLedgerSeq()) return Ledger::pointer();
+
+	// 2) Begin building a new ledger with the specified ledger as previous.
+	Ledger* newLedger=new Ledger(*newPrevious, mTimeStamp);
+
+	// 3) For any transactions in our previous ledger but not in the new previous ledger, add them to the set
+	SHAMap::SHAMapDiff mapDifferences;
+	std::map<uint256, std::pair<Transaction::pointer, Transaction::pointer> > TxnDiff;
+	if(!newPrevious->mTransactionMap->compare(oldPrevious->mTransactionMap, mapDifferences, limit))
+		return Ledger::pointer();
+	if(!Transaction::convertToTransactions(oldPrevious->getLedgerSeq(), newPrevious->getLedgerSeq(),
+			false, true, mapDifferences, TxnDiff))
+		return Ledger::pointer(); // new previous ledger contains invalid transactions
+
+	// 4) Try to add those transactions to the new ledger.
+	do
+	{
+		count=0;
+		std::map<uint256, std::pair<Transaction::pointer, Transaction::pointer> >::iterator it=TxnDiff.begin();
+		while(it!=TxnDiff.end())
+		{
+			Transaction::pointer& tx=it->second.second;
+			if(!tx || newLedger->addTransaction(tx))
+			{
+				count++;
+				TxnDiff.erase(++it);
+			}
+			else ++it;
+		}
+	} while(count!=0);
+
+	// WRITEME: Handle rejected transactions left in TxnDiff
+
+	// 5) Try to add transactions from this ledger to the new ledger.
+	// OPTIMIZME: This should use the transaction canonicalizer, rather than creating transactions
+	std::map<uint256, Transaction::pointer> txnMap;
+	for(SHAMapItem::pointer mit=peekTransactionMap()->peekFirstItem();
+			!!mit;
+			mit=peekTransactionMap()->peekNextItem(mit->getTag()))
+		txnMap.insert(std::make_pair(mit->getTag(), Transaction::pointer(new Transaction(mit->peekData(), false))));
+	do
+	{
+		count=0;
+		std::map<uint256, Transaction::pointer>::iterator it=txnMap.begin();
+		while(it!=txnMap.end())
+		{
+			if(newLedger->addTransaction(it->second))
+			{
+				count++;
+				txnMap.erase(++it);
+			}
+			else ++it;
+		}
+	} while(count!=0);
+
+
+	// WRITEME: Handle rejected transactions left in txnMap
+
+	return Ledger::pointer(newLedger);
+}
diff --git a/Ledger.h b/Ledger.h
index 955fef9c9..953aafa4c 100644
--- a/Ledger.h
+++ b/Ledger.h
@@ -96,6 +96,7 @@ public:
 	TransResult applyTransaction(Transaction::pointer trans);
 	TransResult removeTransaction(Transaction::pointer trans);
 	TransResult hasTransaction(Transaction::pointer trans);
+	Ledger::pointer switchPreviousLedger(Ledger::pointer oldPrevious, Ledger::pointer newPrevious,  int limit);
 
 	// database functions
 	static void saveAcceptedLedger(Ledger::pointer);
diff --git a/SHAMap.cpp b/SHAMap.cpp
index f660b9d99..92f7b2a38 100644
--- a/SHAMap.cpp
+++ b/SHAMap.cpp
@@ -389,9 +389,6 @@ SHAMapLeafNode::pointer SHAMap::createLeaf(const SHAMapInnerNode& lowestParent,
 	for(int depth=lowestParent.getDepth()+1; depth<SHAMapNode::leafDepth; depth++)
 	{
 		SHAMapInnerNode::pointer newNode(new SHAMapInnerNode(SHAMapNode(depth, id), mSeq));
-#ifdef DEBUG
-		std::cerr << "create node " << newNode->getString() << std::endl;
-#endif
 		mInnerNodeByID[*newNode]=newNode;
 	}
 	SHAMapLeafNode::pointer newLeaf(new SHAMapLeafNode(SHAMapNode(SHAMapNode::leafDepth, id), mSeq));
diff --git a/Transaction.cpp b/Transaction.cpp
index 44c2dd229..52c98e3d4 100644
--- a/Transaction.cpp
+++ b/Transaction.cpp
@@ -274,12 +274,10 @@ Transaction::pointer Transaction::findFrom(const uint160& fromID, uint32 seq)
 }
 
 bool Transaction::convertToTransactions(uint32 firstLedgerSeq, uint32 secondLedgerSeq,
-	bool checkFirstTransactions, bool checkSecondTransactions,
-	const std::map<uint256, std::pair<SHAMapItem::pointer, SHAMapItem::pointer> >& inMap,
+	bool checkFirstTransactions, bool checkSecondTransactions, const SHAMap::SHAMapDiff& inMap,
 	std::map<uint256, std::pair<Transaction::pointer, Transaction::pointer> >& outMap)
 { // convert a straight SHAMap payload difference to a transaction difference table
   // return value: true=ledgers are valid, false=a ledger is invalid
-  	bool ret=true;
 	std::map<uint256, std::pair<SHAMapItem::pointer, SHAMapItem::pointer> >::const_iterator it;
 	for(it=inMap.begin(); it!=inMap.end(); ++it)
 	{
@@ -294,7 +292,7 @@ bool Transaction::convertToTransactions(uint32 firstLedgerSeq, uint32 secondLedg
 			if( (firstTrans->getStatus()==INVALID) || (firstTrans->getID()!=id) )
 			{
 				firstTrans->setStatus(INVALID, firstLedgerSeq);
-				ret=false;
+				return false;
 			}
 			else firstTrans->setStatus(INCLUDED, firstLedgerSeq);
 		}
@@ -305,17 +303,17 @@ bool Transaction::convertToTransactions(uint32 firstLedgerSeq, uint32 secondLedg
 			if( (secondTrans->getStatus()==INVALID) || (secondTrans->getID()!=id) )
 			{
 				secondTrans->setStatus(INVALID, secondLedgerSeq);
-				ret=false;
+				return false;
 			}
 			else secondTrans->setStatus(INCLUDED, secondLedgerSeq);
 		}
 		assert(firstTrans || secondTrans);
 		if(firstTrans && secondTrans && (firstTrans->getStatus()!=INVALID) && (secondTrans->getStatus()!=INVALID))
-			ret=false;	// one or the other SHAMap is structurally invalid or a miracle has happened
+			return false; // one or the other SHAMap is structurally invalid or a miracle has happened
 
 		outMap[id]=std::pair<Transaction::pointer, Transaction::pointer>(firstTrans, secondTrans);
 	}
-	return ret;
+	return true;
 }
 
 static bool isHex(char j)
diff --git a/Transaction.h b/Transaction.h
index 7de515084..d11121b04 100644
--- a/Transaction.h
+++ b/Transaction.h
@@ -88,8 +88,7 @@ public:
 
 	// conversion function
 	static bool convertToTransactions(uint32 ourLedgerSeq, uint32 otherLedgerSeq,
-		bool checkFirstTransactions, bool checkSecondTransactions,
-		const std::map<uint256, std::pair<SHAMapItem::pointer,SHAMapItem::pointer> >& inMap,
+		bool checkFirstTransactions, bool checkSecondTransactions, const SHAMap::SHAMapDiff& inMap,
 		std::map<uint256, std::pair<Transaction::pointer, Transaction::pointer> >& outMap);
 
 	bool operator<(const Transaction&) const;