diff --git a/gtk2_ardour/editor.h b/gtk2_ardour/editor.h
index 8b7b2af1d0..e73d44971b 100644
--- a/gtk2_ardour/editor.h
+++ b/gtk2_ardour/editor.h
@@ -1616,7 +1616,7 @@ class Editor : public PublicEditor
 	/* visual history */
 
 	UndoHistory visual_history;
-	UndoCommand current_visual_command;
+	UndoTransaction current_visual_command;
 
 	void begin_reversible_visual_command (const string & cmd_name);
 	void commit_reversible_visual_command ();
diff --git a/libs/ardour/ardour/session.h b/libs/ardour/ardour/session.h
index 83acb3f82a..3d164a4b05 100644
--- a/libs/ardour/ardour/session.h
+++ b/libs/ardour/ardour/session.h
@@ -489,7 +489,7 @@ class Session : public sigc::trackable, public Stateful
 	static vector<string*>* possible_states(string path);
 
 	XMLNode& get_state();
-	int      set_state(const XMLNode& node);
+	int      set_state(const XMLNode& node); // not idempotent
 	XMLNode& get_template();
 
 	void add_instant_xml (XMLNode&, const std::string& dir);
@@ -849,13 +849,13 @@ class Session : public sigc::trackable, public Stateful
 	void commit_reversible_command (UndoAction* private_redo = 0);
 
 	void add_undo (const UndoAction& ua) {
-		current_cmd.add_undo (ua);
+		current_trans.add_undo (ua);
 	}
 	void add_redo (const UndoAction& ua) {
-		current_cmd.add_redo (ua);
+		current_trans.add_redo (ua);
 	}
 	void add_redo_no_execute (const UndoAction& ua) {
-		current_cmd.add_redo_no_execute (ua);
+		current_trans.add_redo_no_execute (ua);
 	}
 
 	UndoAction global_solo_memento (void *src);
@@ -1635,7 +1635,7 @@ class Session : public sigc::trackable, public Stateful
 	void reverse_diskstream_buffers ();
 
 	UndoHistory history;
-	UndoCommand current_cmd;
+	UndoTransaction current_trans;
 
 	GlobalRouteBooleanState get_global_route_boolean (bool (Route::*method)(void) const);
 	GlobalRouteMeterState get_global_route_metering ();
diff --git a/libs/ardour/session_state.cc b/libs/ardour/session_state.cc
index cf510e9881..5ef717de8a 100644
--- a/libs/ardour/session_state.cc
+++ b/libs/ardour/session_state.cc
@@ -2562,11 +2562,11 @@ Session::set_meter_falloff (float val)
 void
 Session::begin_reversible_command (string name, UndoAction* private_undo)
 {
-	current_cmd.clear ();
-	current_cmd.set_name (name);
+	current_trans.clear ();
+	current_trans.set_name (name);
 
 	if (private_undo) {
-		current_cmd.add_undo (*private_undo);
+		current_trans.add_undo (*private_undo);
 	}
 }
 
@@ -2576,13 +2576,13 @@ Session::commit_reversible_command (UndoAction* private_redo)
 	struct timeval now;
 
 	if (private_redo) {
-		current_cmd.add_redo_no_execute (*private_redo);
+		current_trans.add_redo_no_execute (*private_redo);
 	}
 
 	gettimeofday (&now, 0);
-	current_cmd.set_timestamp (now);
+	current_trans.set_timestamp (now);
 
-	history.add (current_cmd);
+	history.add (current_trans);
 }
 
 Session::GlobalRouteBooleanState 
diff --git a/libs/pbd3/pbd/undo.h b/libs/pbd3/pbd/undo.h
index b540179012..488e896706 100644
--- a/libs/pbd3/pbd/undo.h
+++ b/libs/pbd3/pbd/undo.h
@@ -23,6 +23,7 @@
 
 #include <string>
 #include <list>
+#include <map>
 #include <sigc++/slot.h>
 #include <sigc++/bind.h>
 #include <sys/time.h>
@@ -33,42 +34,47 @@ using std::list;
 
 typedef sigc::slot<void> UndoAction;
 
-class Serializable;
-
-class MementoBase
+// TODO stick this in its own file, and make the arguments multiply-inherit it
+class Serializable 
 {
 public:
-    MementoBase(std::string key);
+    XMLNode &serialize();
+};
+
+class UndoCommand
+{
+public:
+    UndoCommand(id_t object_id, std::string method_name);
     void operator() () { return _slot(); }
     XMLNode &serialize();
 protected:
     sigc::slot<void> _slot;
-    std::list<Serializable> _args;
 };
 
 template <class T1=void, class T2=void, class T3=void, class T4=void>
-class Memento;
+class SlotCommand;
 
 template <>
-class Memento <> : public MementoBase {};
+class SlotCommand <> : public UndoCommand {};
 
 template <class T1>
-class Memento <T1> : public MementoBase
+class SlotCommand <T1> : public UndoCommand
 {
+    T1 _arg1;
 public:
-    Memento(std::string key, T1 arg1) : MementoBase(key)
+    SlotCommand(id_t object_id, std::string key, T1 arg1) 
+	: UndoCommand(object_id, key), _arg1(arg1)
     {
-	_args.push_back(arg1);
 	_slot = sigc::bind(_slot, arg1);
     }
 };
 
-class UndoCommand 
+class UndoTransaction 
 {
   public:
-	UndoCommand ();
-	UndoCommand (const UndoCommand&);
-	UndoCommand& operator= (const UndoCommand&);
+	UndoTransaction ();
+	UndoTransaction (const UndoTransaction&);
+	UndoTransaction& operator= (const UndoTransaction&);
 
 	void clear ();
 
@@ -105,7 +111,7 @@ class UndoHistory
 	UndoHistory() {}
 	~UndoHistory() {}
 	
-	void add (UndoCommand uc);
+	void add (UndoTransaction ut);
 	void undo (unsigned int n);
 	void redo (unsigned int n);
 	
@@ -120,8 +126,8 @@ class UndoHistory
 	void clear_redo ();
 
   private:
-	list<UndoCommand> UndoList;
-	list<UndoCommand> RedoList;
+	list<UndoTransaction> UndoList;
+	list<UndoTransaction> RedoList;
 };
 
 
diff --git a/libs/pbd3/undo.cc b/libs/pbd3/undo.cc
index f2f11b1c5c..0aac58effc 100644
--- a/libs/pbd3/undo.cc
+++ b/libs/pbd3/undo.cc
@@ -25,11 +25,11 @@
 using namespace std;
 using namespace sigc;
 
-UndoCommand::UndoCommand ()
+UndoTransaction::UndoTransaction ()
 {
 }
 
-UndoCommand::UndoCommand (const UndoCommand& rhs)
+UndoTransaction::UndoTransaction (const UndoTransaction& rhs)
 {
 	_name = rhs._name;
 	clear ();
@@ -37,8 +37,8 @@ UndoCommand::UndoCommand (const UndoCommand& rhs)
 	redo_actions.insert(redo_actions.end(),rhs.redo_actions.begin(),rhs.redo_actions.end());
 }
 
-UndoCommand& 
-UndoCommand::operator= (const UndoCommand& rhs)
+UndoTransaction& 
+UndoTransaction::operator= (const UndoTransaction& rhs)
 {
 	if (this == &rhs) return *this;
 	_name = rhs._name;
@@ -49,33 +49,33 @@ UndoCommand::operator= (const UndoCommand& rhs)
 }
 
 void
-UndoCommand::add_undo (const UndoAction& action)
+UndoTransaction::add_undo (const UndoAction& action)
 {
 	undo_actions.push_back (action);
 }
 
 void
-UndoCommand::add_redo (const UndoAction& action)
+UndoTransaction::add_redo (const UndoAction& action)
 {
 	redo_actions.push_back (action);
 	redo_actions.back()(); // operator()
 }
 
 void
-UndoCommand::add_redo_no_execute (const UndoAction& action)
+UndoTransaction::add_redo_no_execute (const UndoAction& action)
 {
 	redo_actions.push_back (action);
 }
 
 void
-UndoCommand::clear ()
+UndoTransaction::clear ()
 {
 	undo_actions.clear ();
 	redo_actions.clear ();
 }
 
 void
-UndoCommand::undo ()
+UndoTransaction::undo ()
 {
 	cerr << "Undo " << _name << endl;
 	for (list<UndoAction>::reverse_iterator i = undo_actions.rbegin(); i != undo_actions.rend(); ++i) {
@@ -84,7 +84,7 @@ UndoCommand::undo ()
 }
 
 void
-UndoCommand::redo ()
+UndoTransaction::redo ()
 {
 	cerr << "Redo " << _name << endl;
 	for (list<UndoAction>::iterator i = redo_actions.begin(); i != redo_actions.end(); ++i) {
@@ -93,9 +93,9 @@ UndoCommand::redo ()
 }
 
 void
-UndoHistory::add (UndoCommand uc)
+UndoHistory::add (UndoTransaction ut)
 {
-	UndoList.push_back (uc);
+	UndoList.push_back (ut);
 }
 
 void
@@ -105,10 +105,10 @@ UndoHistory::undo (unsigned int n)
 		if (UndoList.size() == 0) {
 			return;
 		}
-		UndoCommand uc = UndoList.back ();
+		UndoTransaction ut = UndoList.back ();
 		UndoList.pop_back ();
-		uc.undo ();
-		RedoList.push_back (uc);
+		ut.undo ();
+		RedoList.push_back (ut);
 	}
 }
 
@@ -119,10 +119,10 @@ UndoHistory::redo (unsigned int n)
 		if (RedoList.size() == 0) {
 			return;
 		}
-		UndoCommand cmd = RedoList.back ();
+		UndoTransaction trans = RedoList.back ();
 		RedoList.pop_back ();
-		cmd.redo ();
-		UndoList.push_back (cmd);
+		trans.redo ();
+		UndoList.push_back (trans);
 	}
 }