diff --git a/src/db/db/gsiDeclDbLayout.cc b/src/db/db/gsiDeclDbLayout.cc
index 751c4152e..3edf80298 100644
--- a/src/db/db/gsiDeclDbLayout.cc
+++ b/src/db/db/gsiDeclDbLayout.cc
@@ -854,6 +854,11 @@ static db::Cell *cell_from_index (db::Layout *ly, db::cell_index_type ci)
   return &ly->cell (ci);
 }
 
+static const db::Cell *cell_from_index_const (const db::Layout *layout, db::cell_index_type ci)
+{
+  return cell_from_index (const_cast <db::Layout *> (layout), ci);
+}
+
 static db::Cell *cell_from_name (db::Layout *ly, const std::string &name)
 {
   std::pair<bool, db::cell_index_type> cn = ly->cell_by_name (name.c_str ());
@@ -864,6 +869,11 @@ static db::Cell *cell_from_name (db::Layout *ly, const std::string &name)
   }
 }
 
+static const db::Cell *cell_from_name_const (const db::Layout *layout, const std::string &name)
+{
+  return cell_from_name (const_cast <db::Layout *> (layout), name);
+}
+
 static std::vector<db::Cell *> cells_from_name (db::Layout *layout, const std::string &filter)
 {
   tl::GlobPattern gp (filter);
@@ -880,6 +890,12 @@ static std::vector<db::Cell *> cells_from_name (db::Layout *layout, const std::s
   return result;
 }
 
+static std::vector<const db::Cell *> cells_from_name_const (const db::Layout *layout, const std::string &filter)
+{
+  std::vector<db::Cell *> tcs = cells_from_name (const_cast <db::Layout *> (layout), filter);
+  return std::vector<const db::Cell *> (tcs.begin (), tcs.end ());
+}
+
 static std::vector<db::Cell *> top_cells (db::Layout *layout)
 {
   std::vector<db::Cell *> tc;
@@ -891,6 +907,12 @@ static std::vector<db::Cell *> top_cells (db::Layout *layout)
   return tc;
 }
 
+static std::vector<const db::Cell *> top_cells_const (const db::Layout *layout)
+{
+  std::vector<db::Cell *> tcs = top_cells (const_cast <db::Layout *> (layout));
+  return std::vector<const db::Cell *> (tcs.begin (), tcs.end ());
+}
+
 static db::Cell *top_cell (db::Layout *layout)
 {
   db::Cell *tc = 0;
@@ -906,6 +928,11 @@ static db::Cell *top_cell (db::Layout *layout)
   return tc;
 }
 
+static const db::Cell *top_cell_const (const db::Layout *layout)
+{
+  return top_cell (const_cast <db::Layout *> (layout));
+}
+
 static db::Cell *create_cell (db::Layout *layout, const std::string &name)
 {
   return &layout->cell (layout->add_cell (name.c_str ()));
@@ -1328,7 +1355,16 @@ Class<db::Layout> decl_Layout ("db", "Layout",
     "\n"
     "This method has been introduced in version 0.23."
   ) + 
-  gsi::method_ext ("top_cells", &top_cells, 
+  gsi::method_ext ("top_cell", &top_cell_const,
+    "@brief Returns the top cell object (const version)\n"
+    "@return The \\Cell object of the top cell\n"
+    "If the layout has a single top cell, this method returns the top cell's \\Cell object.\n"
+    "If the layout does not have a top cell, this method returns \"nil\". If the layout has multiple\n"
+    "top cells, this method raises an error.\n"
+    "\n"
+    "This variant has been introduced in version 0.29.6."
+  ) +
+  gsi::method_ext ("top_cells", &top_cells,
     "@brief Returns the top cell objects\n"
     "@return The \\Cell objects of the top cells\n"
     "This method returns and array of \\Cell objects representing the top cells of the layout.\n"
@@ -1336,6 +1372,14 @@ Class<db::Layout> decl_Layout ("db", "Layout",
     "\n"
     "This method has been introduced in version 0.23."
   ) + 
+  gsi::method_ext ("top_cells", &top_cells_const,
+    "@brief Returns the top cell objects (const version)\n"
+    "@return The \\Cell objects of the top cells\n"
+    "This method returns and array of \\Cell objects representing the top cells of the layout.\n"
+    "This array can be empty, if the layout does not have a top cell (i.e. no cell at all).\n"
+    "\n"
+    "This variant has been introduced in version 0.29.6."
+  ) +
   gsi::method ("has_cell?", &db::Layout::has_cell, gsi::arg ("name"),
     "@brief Returns true if a cell with a given name exists\n"
     "Returns true, if the layout has a cell with the given name"
@@ -1780,6 +1824,16 @@ Class<db::Layout> decl_Layout ("db", "Layout",
     "\n"
     "This method has been introduced in version 0.27.3.\n"
   ) +
+  gsi::method_ext ("cells", &cells_from_name_const, gsi::arg ("name_filter"),
+    "@brief Gets the cell objects for a given name filter (const version)\n"
+    "\n"
+    "@param name_filter The cell name filter (glob pattern)\n"
+    "@return A list of \\Cell object of the cells matching the pattern\n"
+    "\n"
+    "This method has been introduced in version 0.27.3.\n"
+    "\n"
+    "This variant has been introduced in version 0.29.6."
+  ) +
   gsi::method_ext ("cell", &cell_from_name, gsi::arg ("name"),
     "@brief Gets a cell object from the cell name\n"
     "\n"
@@ -1789,6 +1843,17 @@ Class<db::Layout> decl_Layout ("db", "Layout",
     "If name is not a valid cell name, this method will return \"nil\".\n"
     "This method has been introduced in version 0.23 and replaces \\cell_by_name.\n"
   ) +
+  gsi::method_ext ("cell", &cell_from_name_const, gsi::arg ("name"),
+    "@brief Gets a cell object from the cell name (const version)\n"
+    "\n"
+    "@param name The cell name\n"
+    "@return A reference to the cell (a \\Cell object)\n"
+    "\n"
+    "If name is not a valid cell name, this method will return \"nil\".\n"
+    "This method has been introduced in version 0.23 and replaces \\cell_by_name.\n"
+    "\n"
+    "This variant has been introduced in version 0.29.6."
+  ) +
   gsi::method_ext ("cell", &cell_from_index, gsi::arg ("i"),
     "@brief Gets a cell object from the cell index\n"
     "\n"
@@ -1798,6 +1863,17 @@ Class<db::Layout> decl_Layout ("db", "Layout",
     "If the cell index is not a valid cell index, this method will raise an error. "
     "Use \\is_valid_cell_index? to test whether a given cell index is valid.\n"
   ) +
+  gsi::method_ext ("cell", &cell_from_index_const, gsi::arg ("i"),
+    "@brief Gets a cell object from the cell index (const version)\n"
+    "\n"
+    "@param i The cell index\n"
+    "@return A reference to the cell (a \\Cell object)\n"
+    "\n"
+    "If the cell index is not a valid cell index, this method will raise an error. "
+    "Use \\is_valid_cell_index? to test whether a given cell index is valid.\n"
+    "\n"
+    "This variant has been introduced in version 0.29.6."
+  ) +
   gsi::iterator ("each_cell", (db::Layout::iterator (db::Layout::*) ()) &db::Layout::begin, (db::Layout::iterator (db::Layout::*) ()) &db::Layout::end,
     "@brief Iterates the unsorted cell list\n"
   ) +
diff --git a/src/db/db/gsiDeclDbLibrary.cc b/src/db/db/gsiDeclDbLibrary.cc
index a476dfba0..69eb095bc 100644
--- a/src/db/db/gsiDeclDbLibrary.cc
+++ b/src/db/db/gsiDeclDbLibrary.cc
@@ -339,7 +339,7 @@ Class<db::ParameterState> decl_PCellParameterState ("db", "PCellParameterState",
   gsi::method("icon=", &db::ParameterState::set_icon, gsi::arg ("i"),
     "@brief Sets the icon for the parameter\n"
   ) +
-  gsi::method("tooltip", &db::ParameterState::tooltip,
+  gsi::method("icon", &db::ParameterState::icon,
     "@brief Gets the icon for the parameter\n"
   ),
   "@brief Provides access to the attributes of a single parameter within \\PCellParameterStates.\n"
diff --git a/src/drc/drc/built-in-macros/_drc_engine.rb b/src/drc/drc/built-in-macros/_drc_engine.rb
index bcf27580b..176bf9aaa 100644
--- a/src/drc/drc/built-in-macros/_drc_engine.rb
+++ b/src/drc/drc/built-in-macros/_drc_engine.rb
@@ -2308,7 +2308,7 @@ def #{f}(*args)
  
     def netter
       self._context("netter") do
-        DRC::DRCNetter::new
+        DRC::DRCNetter::new(self)
       end
     end
 
diff --git a/src/drc/drc/built-in-macros/_drc_layer.rb b/src/drc/drc/built-in-macros/_drc_layer.rb
index b06b04e77..5cf3ecd8b 100644
--- a/src/drc/drc/built-in-macros/_drc_layer.rb
+++ b/src/drc/drc/built-in-macros/_drc_layer.rb
@@ -5186,6 +5186,7 @@ def map_props(arg)
     # @brief Pulls net shapes from selected or all nets, optionally annotating nets with properties
     # @synopsis layer.nets
     # @synopsis layer.nets(net_filter)
+    # @synopsis layer.nets(net_object)
     # @synopsis layer.nets(circuit_filter, net_filter)
     # @synopsis layer.nets(netter, ...)
     # @synopsis layer.nets(prop(key), ...)
@@ -5202,7 +5203,7 @@ def map_props(arg)
     # complete - subnets from subcircuits are not selected. The net name is taken from
     # the net's home circuit (to topmost location where all net connections are formed).
     # You can specify a circuit filter to select nets from certain circuits only or
-    # give a RBA::Circuit object explicitly. 
+    # give a RBA::Circuit object explicitly. You can also specify RBA::Net objects directly.
     # 
     # @code
     # connect(metal1, via1)
@@ -5230,6 +5231,8 @@ def nets(*args)
 
       @engine._context("nets") do
 
+        nets = nil
+
         # parse arguments
         filters = nil
         circuits = nil
@@ -5241,6 +5244,9 @@ def nets(*args)
             filters << a
           elsif a.is_a?(1.class)
             prop_id = a
+          elsif a.is_a?(RBA::Net)
+            nets ||= []
+            nets << a
           elsif a.is_a?(RBA::Circuit)
             circuits ||= []
             circuits << a
@@ -5272,13 +5278,14 @@ def nets(*args)
           circuits ||= []
           circuits += netlist.circuits_by_name(circuit_filter)
         end
-        nets = nil
         if !circuits
           if filters
-            nets = filters.collect { |f| netlist.nets_by_name(f) }.flatten
+            nets ||= []
+            nets += filters.collect { |f| netlist.nets_by_name(f) }.flatten
           end
         else
-          nets = circuits.collect do |circuit|
+          nets ||= []
+          nets += circuits.collect do |circuit|
             (filters || ["*"]).collect { |f| circuit.nets_by_name(f) }.flatten
           end.flatten
         end
diff --git a/src/drc/unit_tests/drcSimpleTests.cc b/src/drc/unit_tests/drcSimpleTests.cc
index a561ad241..31ae49654 100644
--- a/src/drc/unit_tests/drcSimpleTests.cc
+++ b/src/drc/unit_tests/drcSimpleTests.cc
@@ -1568,6 +1568,16 @@ TEST(70d_props)
   run_test (_this, "70", true);
 }
 
+TEST(71_netter)
+{
+  run_test (_this, "71", false);
+}
+
+TEST(71d_netter)
+{
+  run_test (_this, "71", true);
+}
+
 TEST(80_deep_with_mag_width)
 {
   run_test (_this, "80", true);
diff --git a/src/gsi/gsi/gsiClassBase.cc b/src/gsi/gsi/gsiClassBase.cc
index c9824a46d..6e75b461e 100644
--- a/src/gsi/gsi/gsiClassBase.cc
+++ b/src/gsi/gsi/gsiClassBase.cc
@@ -353,6 +353,48 @@ sm_is_const (const char *name)
   return sm;
 }
 
+static SpecialMethod *
+sm_to_const (const char *name, const gsi::ClassBase *cls)
+{
+  SpecialMethod *sm = new SpecialMethod (name,
+    tl::to_string (tr ("@hide")),  //  provided for test purposes mainly
+    true,    //  const
+    false,   //  non-static
+    MethodBase::ToConst);
+
+  gsi::ArgType ret;
+  ret.set_is_cptr (true);
+  ret.set_type (gsi::T_object);
+  ret.set_pass_obj (false);
+  ret.set_cls (cls);
+  sm->set_return (ret);
+
+  return sm;
+}
+
+static SpecialMethod *
+sm_const_cast (const char *name, const gsi::ClassBase *cls)
+{
+  SpecialMethod *sm = new SpecialMethod (name,
+    tl::to_string (tr ("@brief Returns a non-const reference to self.\n"
+                       "Basically, this method allows turning a const object reference to a non-const one. "
+                       "This method is provided as last resort to remove the constness from an object. Usually there is a good reason for a const object reference, so using this method may have undesired side effects.\n"
+                       "\n"
+                       "This method has been introduced in version 0.29.6.")),
+    true,    //  const
+    false,   //  non-static
+    MethodBase::ConstCast);
+
+  gsi::ArgType ret;
+  ret.set_is_ptr (true);
+  ret.set_type (gsi::T_object);
+  ret.set_pass_obj (false);
+  ret.set_cls (cls);
+  sm->set_return (ret);
+
+  return sm;
+}
+
 static SpecialMethod *
 sm_destroyed (const char *name)
 {
@@ -598,6 +640,9 @@ ClassBase::merge_declarations ()
       non_const_decl->add_method (sm_is_const ("_is_const_object?"));
     }
 
+    non_const_decl->add_method (sm_to_const ("_to_const_object", &*c));
+    non_const_decl->add_method (sm_const_cast ("_const_cast", &*c));
+
   }
 
   //  finally merge the new classes into the existing ones
diff --git a/src/gsi/gsi/gsiExpression.cc b/src/gsi/gsi/gsiExpression.cc
index 46ca33374..58795abd4 100644
--- a/src/gsi/gsi/gsiExpression.cc
+++ b/src/gsi/gsi/gsiExpression.cc
@@ -305,13 +305,17 @@ initialize_expressions ()
     //  install the method table:
     ExpressionMethodTable::initialize_class (*c);
 
-    //  register a function that creates a class object (use a function to avoid issues with
-    //  late destruction of global variables which the class object is already gone)
-    const tl::VariantUserClassBase *cc = (*c)->var_cls_cls ();
-    if (cc) {
-      tl::Eval::define_global_function ((*c)->name (), new EvalClassFunction (cc));
-    }
+    //  Note: skip non-top-level classes
+    if ((*c)->parent () == 0) {
+
+      //  register a function that creates a class object (use a function to avoid issues with
+      //  late destruction of global variables which the class object is already gone)
+      const tl::VariantUserClassBase *cc = (*c)->var_cls_cls ();
+      if (cc) {
+        tl::Eval::define_global_function ((*c)->name (), new EvalClassFunction (cc));
+      }
 
+    }
   }
 }
 
@@ -630,6 +634,14 @@ special_method_impl (gsi::MethodBase::special_method_type smt, tl::Variant &self
     //  nothing to do here for GSI objects
   } else if (smt == gsi::MethodBase::IsConst) {
     return tl::Variant (self.user_is_const ());
+  } else if (smt == gsi::MethodBase::ToConst) {
+    tl::Variant res (self);
+    res.user_change_constness (true);
+    return res;
+  } else if (smt == gsi::MethodBase::ConstCast) {
+    tl::Variant res (self);
+    res.user_change_constness (false);
+    return res;
   } else if (smt == gsi::MethodBase::Destroyed) {
 
     if (self.type_code () == tl::Variant::t_user) {
diff --git a/src/gsi/gsi/gsiMethods.h b/src/gsi/gsi/gsiMethods.h
index e807ad955..d399ff309 100644
--- a/src/gsi/gsi/gsiMethods.h
+++ b/src/gsi/gsi/gsiMethods.h
@@ -83,7 +83,9 @@ class GSI_PUBLIC MethodBase
     Destroy,
     Create,
     IsConst, 
-    Destroyed, 
+    ConstCast,
+    ToConst,
+    Destroyed,
     Assign, 
     Dup
   };
diff --git a/src/gsi/unit_tests/gsiExpressionTests.cc b/src/gsi/unit_tests/gsiExpressionTests.cc
index d85f566fe..16c7f4dfd 100644
--- a/src/gsi/unit_tests/gsiExpressionTests.cc
+++ b/src/gsi/unit_tests/gsiExpressionTests.cc
@@ -829,3 +829,37 @@ TEST(15)
   v = e.parse("var bb = BB.new; bb.d4(1, 'a', 2.0, BB.E.E3B, 42)").execute();
   EXPECT_EQ (v.to_string (), "1,a,2,101,42");
 }
+
+//  constness
+TEST(16)
+{
+  tl::Eval e;
+  tl::Variant v;
+  v = e.parse ("var b=B.new(); b._is_const_object").execute ();
+  EXPECT_EQ (v.to_string (), std::string ("false"));
+  try {
+    v = e.parse ("var b=B.new(); var bc=b._to_const_object; bc.set_str('abc')").execute ();
+    EXPECT_EQ (1, 0);
+  } catch (tl::Exception &ex) {
+    EXPECT_EQ (ex.msg (), "Cannot call non-const method set_str, class B on a const reference at position 44 (...set_str('abc'))");
+  }
+  v = e.parse ("var e=E.new(); var ec=e.dup; [e._is_const_object, ec._to_const_object._is_const_object]").execute ();
+  EXPECT_EQ (v.to_string (), std::string ("false,true"));
+  v = e.parse ("var e=E.new(); var ec=e._to_const_object; e.x=17; [e.x, ec.x]").execute ();
+  EXPECT_EQ (v.to_string (), std::string ("17,17"));
+  v = e.parse ("var e=E.new(); var ec=e._to_const_object; ec._is_const_object").execute ();
+  EXPECT_EQ (v.to_string (), std::string ("true"));
+  v = e.parse ("var e=E.new(); var ec=e._to_const_object; ec=ec._const_cast; ec._is_const_object").execute ();
+  EXPECT_EQ (v.to_string (), std::string ("false"));
+  v = e.parse ("var e=E.new(); var ec=e._to_const_object; ec=ec._const_cast; ec.x=42; e.x").execute ();
+  EXPECT_EQ (v.to_string (), std::string ("42"));
+  v = e.parse ("var e=E.new(); var ec=e._to_const_object; e.x=17; ec.x").execute ();
+  EXPECT_EQ (v.to_string (), std::string ("17"));
+  try {
+    v = e.parse ("var e=E.new(); var ec=e._to_const_object; e.x=17; e._destroy; ec.x").execute ();
+    EXPECT_EQ (1, 0);
+  } catch (tl::Exception &ex) {
+    EXPECT_EQ (ex.msg (), "Object has been destroyed already at position 64 (...x)");
+  }
+}
+
diff --git a/src/lay/lay/layMainWindow.cc b/src/lay/lay/layMainWindow.cc
index dce7dd237..c60b09921 100644
--- a/src/lay/lay/layMainWindow.cc
+++ b/src/lay/lay/layMainWindow.cc
@@ -1331,6 +1331,18 @@ MainWindow::update_dock_widget_state ()
 void
 MainWindow::exit ()
 {
+  if (m_busy) {
+    if (QMessageBox::warning (this,
+      QObject::tr ("Application Busy - Close Anyway?"),
+      QObject::tr ("The application is busy.\nYou can close the application now, but this will terminate any running operations.\nDo you want to close anyway?\n\n"
+                   "Press 'Yes' to end the application now."),
+      QMessageBox::Yes | QMessageBox::No,
+      QMessageBox::Yes) != QMessageBox::Yes)
+    {
+      return;
+    }
+  }
+
   m_exited = true;
 
   //  If there is a operation ongoing, request a break and delay execution of the exit operation.
@@ -1394,20 +1406,6 @@ MainWindow::dirty_files (std::string &dirty_files)
 bool
 MainWindow::can_close ()
 {
-  if (m_busy) {
-
-    bool can_close = false;
-
-    can_close = (QMessageBox::warning (this,
-      QObject::tr ("Application Busy"),
-      QObject::tr ("The application is busy.\nYou can close the application now, but any unsaved data will be lost.\n\nPress 'Yes' to end the application now."),
-      QMessageBox::Yes | QMessageBox::No,
-      QMessageBox::Yes) == QMessageBox::Yes);
-
-    return can_close;
-
-  }
-
   for (tl::Registrar<lay::PluginDeclaration>::iterator cls = tl::Registrar<lay::PluginDeclaration>::begin (); cls != tl::Registrar<lay::PluginDeclaration>::end (); ++cls) {
     lay::PluginDeclaration *pd = const_cast<lay::PluginDeclaration *> (&*cls);
     if (! pd->can_exit (dispatcher ())) {
diff --git a/src/layui/layui/layPropertiesDialog.cc b/src/layui/layui/layPropertiesDialog.cc
index 8a0fbe274..0e794983f 100644
--- a/src/layui/layui/layPropertiesDialog.cc
+++ b/src/layui/layui/layPropertiesDialog.cc
@@ -379,7 +379,7 @@ PropertiesDialog::update_controls ()
 
   mp_ui->apply_to_all_cbx->setChecked (m_object_indexes.size () > 1);
 
-  if (m_index < 0) {
+  if (m_index < 0 || m_index >= int (mp_properties_pages.size ())) {
 
     mp_stack->setCurrentWidget (mp_none);
 
@@ -552,6 +552,10 @@ PropertiesDialog::apply ()
 {
 BEGIN_PROTECTED
 
+  if (m_index < 0 || m_index >= int (mp_properties_pages.size ())) {
+    return;
+  }
+
   db::Transaction t (mp_manager, tl::to_string (QObject::tr ("Apply changes")), m_transaction_id);
 
   try {
@@ -633,6 +637,12 @@ PropertiesDialog::reject ()
   QDialog::reject ();
 }
 
+void
+PropertiesDialog::accept ()
+{
+  //  stop handling "Enter" key.
+}
+
 }
 
 #endif
diff --git a/src/layui/layui/layPropertiesDialog.h b/src/layui/layui/layPropertiesDialog.h
index 514f0d502..67792595f 100644
--- a/src/layui/layui/layPropertiesDialog.h
+++ b/src/layui/layui/layPropertiesDialog.h
@@ -116,6 +116,7 @@ public slots:
 
 protected:
   void reject ();
+  void accept ();
 
 private:
   Ui::PropertiesDialog *mp_ui;
diff --git a/src/pya/pya/pyaCallables.cc b/src/pya/pya/pyaCallables.cc
index 5422c9968..5dfc3e4f1 100644
--- a/src/pya/pya/pyaCallables.cc
+++ b/src/pya/pya/pyaCallables.cc
@@ -721,6 +721,50 @@ object_is_const (PyObject *self, PyObject *args)
   return c2python (PYAObjectBase::from_pyobject (self)->const_ref ());
 }
 
+/**
+ *  @brief Implements to_const and const_cast
+ */
+static PyObject *
+object_change_const (PyObject *self, PyObject *args, bool to_const)
+{
+  if (PYAObjectBase::from_pyobject (self)->const_ref () == to_const) {
+    return self;
+  }
+
+  const gsi::ClassBase *cls_decl_self = PythonModule::cls_for_type (Py_TYPE (self));
+  tl_assert (cls_decl_self != 0);
+
+  if (! PyArg_ParseTuple (args, "")) {
+    return NULL;
+  }
+
+  PyObject *new_object = Py_TYPE (self)->tp_alloc (Py_TYPE (self), 0);
+  PythonRef obj (new_object);
+  PYAObjectBase *new_pya_base = PYAObjectBase::from_pyobject_unsafe (new_object);
+  new (new_pya_base) PYAObjectBase (cls_decl_self, new_object);
+  new_pya_base->set (PYAObjectBase::from_pyobject (self)->obj (), false, to_const, false);
+
+  return obj.release ();
+}
+
+/**
+ *  @brief Implements to_const
+ */
+static PyObject *
+object_to_const (PyObject *self, PyObject *args)
+{
+  return object_change_const (self, args, true);
+}
+
+/**
+ *  @brief Implements const_cast
+ */
+static PyObject *
+object_const_cast (PyObject *self, PyObject *args)
+{
+  return object_change_const (self, args, false);
+}
+
 static PyObject *
 special_method_impl (gsi::MethodBase::special_method_type smt, PyObject *self, PyObject *args)
 {
@@ -734,6 +778,10 @@ special_method_impl (gsi::MethodBase::special_method_type smt, PyObject *self, P
     return object_create (self, args);
   } else if (smt == gsi::MethodBase::IsConst) {
     return object_is_const (self, args);
+  } else if (smt == gsi::MethodBase::ToConst) {
+    return object_to_const (self, args);
+  } else if (smt == gsi::MethodBase::ConstCast) {
+    return object_const_cast (self, args);
   } else if (smt == gsi::MethodBase::Destroyed) {
     return object_destroyed (self, args);
   } else if (smt == gsi::MethodBase::Assign) {
diff --git a/src/rba/rba/rba.cc b/src/rba/rba/rba.cc
index 21743409b..efda4fce5 100644
--- a/src/rba/rba/rba.cc
+++ b/src/rba/rba/rba.cc
@@ -913,6 +913,16 @@ handle_exception (const std::string &where)
     handle_exception ((where)); \
   }
 
+static void free_proxy (void *p)
+{
+  delete ((Proxy *) p);
+}
+
+static void mark_proxy (void *p)
+{
+  ((Proxy *) p)->mark ();
+}
+
 static VALUE
 destroy (VALUE self)
 {
@@ -971,6 +981,37 @@ is_const (VALUE self)
   return c2ruby<bool> (p->const_ref ());
 }
 
+static VALUE
+to_const (VALUE self)
+{
+  Proxy *p = 0;
+  Data_Get_Struct (self, Proxy, p);
+  if (! p->const_ref ()) {
+    //  promote to const object
+    //  NOTE: there is only ONE instance we're going to change this instance
+    //  to const here. This has a global effect and this is the reason why this
+    //  method is not public. It is provided for testing purposes mainly.
+    p->set_const_ref (true);
+  }
+
+  return self;
+}
+
+static VALUE
+const_cast_ (VALUE self)
+{
+  Proxy *p = 0;
+  Data_Get_Struct (self, Proxy, p);
+  if (p->const_ref ()) {
+    //  promote to non-const object
+    //  NOTE: this is a global change of constness and will affect all references
+    //  that exist for this object.
+    p->set_const_ref (false);
+  }
+
+  return self;
+}
+
 static VALUE
 assign (VALUE self, VALUE src)
 {
@@ -1024,6 +1065,12 @@ special_method_impl (const gsi::MethodBase *meth, int argc, VALUE *argv, VALUE s
   } else if (smt == gsi::MethodBase::IsConst) {
     tl_assert (!ctor);
     return is_const (self);
+  } else if (smt == gsi::MethodBase::ToConst) {
+    tl_assert (!ctor);
+    return to_const (self);
+  } else if (smt == gsi::MethodBase::ConstCast) {
+    tl_assert (!ctor);
+    return const_cast_ (self);
   } else if (smt == gsi::MethodBase::Destroyed) {
     tl_assert (!ctor);
     return destroyed (self);
@@ -1043,16 +1090,6 @@ special_method_impl (const gsi::MethodBase *meth, int argc, VALUE *argv, VALUE s
   }
 }
 
-static void free_proxy (void *p)
-{
-  delete ((Proxy *) p);
-}
-
-static void mark_proxy (void *p)
-{
-  ((Proxy *) p)->mark ();
-}
-
 static VALUE alloc_proxy (VALUE klass)
 {
   tl_assert (TYPE (klass) == T_CLASS);
diff --git a/src/tl/tl/tlExpression.cc b/src/tl/tl/tlExpression.cc
index 0db330f5c..35eaa01a4 100644
--- a/src/tl/tl/tlExpression.cc
+++ b/src/tl/tl/tlExpression.cc
@@ -3192,6 +3192,17 @@ Eval::set_var (const std::string &name, const tl::Variant &var)
   m_local_vars.insert (std::make_pair (name, tl::Variant ())).first->second = var;
 }
 
+tl::Variant *
+Eval::var (const std::string &name)
+{
+  auto f = m_local_vars.find (name);
+  if (f != m_local_vars.end ()) {
+    return &f->second;
+  } else {
+    return 0;
+  }
+}
+
 void 
 Eval::define_function (const std::string &name, EvalFunction *function)
 {
@@ -3202,6 +3213,17 @@ Eval::define_function (const std::string &name, EvalFunction *function)
   f = function;
 }
 
+EvalFunction *
+Eval::function (const std::string &name)
+{
+  auto f = m_local_functions.find (name);
+  if (f != m_local_functions.end ()) {
+    return f->second;
+  } else {
+    return 0;
+  }
+}
+
 void
 Eval::eval_top (ExpressionParserContext &ex, std::unique_ptr<ExpressionNode> &n)
 {
diff --git a/src/tl/tl/tlExpression.h b/src/tl/tl/tlExpression.h
index 9c7d84d4e..3912d0a5b 100644
--- a/src/tl/tl/tlExpression.h
+++ b/src/tl/tl/tlExpression.h
@@ -448,6 +448,12 @@ class TL_PUBLIC Eval
    */
   void define_function (const std::string &name, EvalFunction *function);
 
+  /**
+   *  @brief Gets the function for the given name
+   *  Returns 0 if there is no such function.
+   */
+  EvalFunction *function (const std::string &name);
+
   /**
    *  @brief Define a global variable for use within an expression
    */
@@ -461,6 +467,12 @@ class TL_PUBLIC Eval
    */
   void set_var (const std::string &name, const tl::Variant &var);
 
+  /**
+   *  @brief Gets the function for the given name
+   *  Returns 0 if there is no such function.
+   */
+  tl::Variant *var (const std::string &name);
+
   /**
    *  @brief Parse an expression from the extractor
    *
@@ -543,6 +555,30 @@ class TL_PUBLIC Eval
     return m_match_substrings;
   }
 
+  /**
+   *  @brief Gets the global context
+   */
+  static tl::Eval &global_context ()
+  {
+    return m_global;
+  }
+
+  /**
+   *  @brief Gets the global context for this context
+   */
+  tl::Eval *global ()
+  {
+    return mp_global;
+  }
+
+  /**
+   *  @brief Gets the parent context for this context
+   */
+  tl::Eval *parent ()
+  {
+    return mp_parent;
+  }
+
 private:
   friend class Expression;
 
diff --git a/src/tl/tl/tlVariant.cc b/src/tl/tl/tlVariant.cc
index 5bdca8fec..68f7240e9 100644
--- a/src/tl/tl/tlVariant.cc
+++ b/src/tl/tl/tlVariant.cc
@@ -2738,6 +2738,17 @@ void *Variant::user_unshare () const
   return const_cast<void *> (to_user ());
 }
 
+void Variant::user_change_constness (bool constness)
+{
+  tl_assert (is_user ());
+
+  if (m_type == t_user) {
+    m_var.mp_user.cls = m_var.mp_user.cls->change_constness (constness);
+  } else if (m_type == t_user_ref) {
+    m_var.mp_user_ref.cls = m_var.mp_user_ref.cls->change_constness (constness);
+  }
+}
+
 void Variant::user_assign (const tl::Variant &other)
 {
   tl_assert (is_user ());
diff --git a/src/tl/tl/tlVariant.h b/src/tl/tl/tlVariant.h
index 9a81c329d..8669f5c82 100644
--- a/src/tl/tl/tlVariant.h
+++ b/src/tl/tl/tlVariant.h
@@ -89,6 +89,7 @@ class TL_PUBLIC VariantUserClassBase
   virtual const gsi::ClassBase *gsi_cls () const = 0;
   virtual const tl::EvalClass *eval_cls () const = 0;
   virtual void *deref_proxy (tl::Object *proxy) const = 0;
+  virtual const tl::VariantUserClassBase *change_constness (bool constness) const = 0;
 
   const void *deref_proxy_const (const tl::Object *proxy) const
   {
@@ -126,6 +127,11 @@ class TL_PUBLIC_TEMPLATE VariantUserClass
     return VariantUserClassBase::instance (typeid (T), is_const);
   }
 
+  const tl::VariantUserClassBase *change_constness (bool constness) const
+  {
+    return instance (constness);
+  }
+
 private:
   static const tl::VariantUserClassBase *ms_instances[4];
 
@@ -1001,6 +1007,11 @@ class TL_PUBLIC Variant
    */
   void *user_unshare () const;
 
+  /**
+   *  @brief Changes the constness of the user object
+   */
+  void user_change_constness (bool constness);
+
   /**
    *  @brief Assigns the object stored in other to self
    *
diff --git a/src/tl/unit_tests/tlExpressionTests.cc b/src/tl/unit_tests/tlExpressionTests.cc
index 3f4710ad4..f00ed10be 100644
--- a/src/tl/unit_tests/tlExpressionTests.cc
+++ b/src/tl/unit_tests/tlExpressionTests.cc
@@ -465,6 +465,7 @@ class BoxClassClass : public tl::VariantUserClassBase, private tl::EvalClass
   virtual bool is_ref () const { return false; }
   virtual void *deref_proxy (tl::Object *) const { return 0; }
   virtual const gsi::ClassBase*gsi_cls() const { return 0; }
+  virtual const tl::VariantUserClassBase *change_constness (bool) const { return this; }
   static BoxClassClass instance;
 };
 
@@ -533,6 +534,7 @@ class EdgeClassClass : public tl::VariantUserClassBase, private tl::EvalClass
   virtual bool is_ref () const { return false; }
   virtual void *deref_proxy (tl::Object *) const { return 0; }
   virtual const gsi::ClassBase*gsi_cls() const { return 0; }
+  virtual const tl::VariantUserClassBase *change_constness (bool) const { return this; }
   static EdgeClassClass instance;
 };
 
@@ -1209,3 +1211,4 @@ TEST(20)
   v = e.parse ("i2.dtrans.disp.y").execute ();
   EXPECT_EQ (v.to_string (), std::string ("0.3"));
 }
+
diff --git a/testdata/drc/drcSimpleTests_70.drc b/testdata/drc/drcSimpleTests_70.drc
index c39b4556f..b46ad66e4 100644
--- a/testdata/drc/drcSimpleTests_70.drc
+++ b/testdata/drc/drcSimpleTests_70.drc
@@ -68,11 +68,14 @@ l1.nets(self._netter).output(101, 0)
 l1.nets(prop(1)).output(102, 0)
 l1.nets(prop(nil)).output(103, 0)
 
-l1.nets("X").output(110, 0)
+x1 = l1.nets("X")
+x1.output(110, 0)
 l1.nets("TOP", "X").output(111, 0)
 l1.nets("TOP", "NOTEXIST").output(112, 0)
 l1.nets("NOTEXIST", "NOTEXIST").output(113, 0)
 
+x2 = l1.nets(*netlist.nets_by_name("X"))
+(x1 ^ x2).output(114, 0)
 
 # checks with property constraints
 
diff --git a/testdata/drc/drcSimpleTests_71.drc b/testdata/drc/drcSimpleTests_71.drc
new file mode 100644
index 000000000..e5d6dbf65
--- /dev/null
+++ b/testdata/drc/drcSimpleTests_71.drc
@@ -0,0 +1,41 @@
+
+# Moved implementation
+
+source($drc_test_source)
+target($drc_test_target)
+
+if $drc_test_deep
+  deep
+end
+
+l1 = input(1, 0)
+l2 = input(2, 0)
+l3 = input(3, 0)
+
+# dump to output
+
+l1.output(1, 0)
+l2.output(2, 0)
+l3.output(3, 0)
+
+ctx1 = netter
+ctx1.connect(l1, l2)
+
+connect(l1, l3)
+
+l1.nets.output(100, 0)
+begin
+  l2.nets.output(101, 0)
+  raise "internal error - l2 was not used in connectivity"
+rescue => ex
+end
+l3.nets.output(102, 0)
+
+l1.nets(ctx1).output(110, 0)
+l2.nets(ctx1).output(111, 0)
+begin
+  l3.nets(ctx1).output(112, 0)
+  raise "internal error - l3 was not used in connectivity"
+rescue => ex
+end
+
diff --git a/testdata/drc/drcSimpleTests_71.gds b/testdata/drc/drcSimpleTests_71.gds
new file mode 100644
index 000000000..d2cac040a
Binary files /dev/null and b/testdata/drc/drcSimpleTests_71.gds differ
diff --git a/testdata/drc/drcSimpleTests_au71.gds b/testdata/drc/drcSimpleTests_au71.gds
new file mode 100644
index 000000000..389d5ccac
Binary files /dev/null and b/testdata/drc/drcSimpleTests_au71.gds differ
diff --git a/testdata/drc/drcSimpleTests_au71d.gds b/testdata/drc/drcSimpleTests_au71d.gds
new file mode 100644
index 000000000..10559625a
Binary files /dev/null and b/testdata/drc/drcSimpleTests_au71d.gds differ
diff --git a/testdata/python/basic.py b/testdata/python/basic.py
index fe720a451..238e6822d 100644
--- a/testdata/python/basic.py
+++ b/testdata/python/basic.py
@@ -3280,6 +3280,44 @@ def test_92(self):
     self.assertEqual(bb.d4(1, "a", d=pya.BB.E.E3B, c=2.5), "1,a,2.5,101,nil")
     self.assertEqual(bb.d4(1, "a", 2.0, pya.BB.E.E3B, 42), "1,a,2,101,42")
 
+  # constness
+  def test_93(self):
+
+    b = pya.B()
+    self.assertEqual(b.is_const_object(), False)
+
+    bc = b._to_const_object()
+    self.assertEqual(bc.is_const_object(), True)
+
+    m = ""
+    try:
+      bc.set_str("abc")
+      self.assertEqual(1, 0)
+    except Exception as ex:
+      m = str(ex)
+
+    self.assertEqual(m, "Cannot call non-const method on a const reference in B.set_str")
+
+    b = pya.B()
+    bc = b
+    self.assertEqual(b._is_const_object(), False)
+    self.assertEqual(bc._is_const_object(), False)
+    bc = bc._to_const_object()
+    b.set_str("abc")
+    self.assertEqual(b._is_const_object(), False)
+    self.assertEqual(bc._is_const_object(), True)
+    self.assertEqual(b.str(), "abc")
+    self.assertEqual(bc.str(), "abc")
+
+    bnc = bc._const_cast()
+    self.assertEqual(b._is_const_object(), False)
+    self.assertEqual(bc._is_const_object(), True)
+    self.assertEqual(bnc._is_const_object(), False)
+    bnc.set_str("xyz")
+    self.assertEqual(b.str(), "xyz")
+    self.assertEqual(bc.str(), "xyz")
+    self.assertEqual(bnc.str(), "xyz")
+
 # run unit tests
 if __name__ == '__main__':
   suite = unittest.TestSuite()
diff --git a/testdata/python/dbLayoutTest.py b/testdata/python/dbLayoutTest.py
index b0e535b7a..1e01a3533 100644
--- a/testdata/python/dbLayoutTest.py
+++ b/testdata/python/dbLayoutTest.py
@@ -75,13 +75,20 @@ def test_2_Layout(self):
     self.assertEqual( ly.cell_name(ci), "new_cell" )
 
     self.assertEqual( ly.cell_by_name("new_cell"), ci )
+    self.assertEqual( ly.cell(ci).cell_index(), ci )
     self.assertEqual( ly.cell("new_cell").name, "new_cell" )
     self.assertEqual( repr(ly.cell("x")), "None" )
+    lyc = ly.dup()
+    self.assertEqual( lyc._to_const_object().cell("new_cell").name, "new_cell" )
+    self.assertEqual( lyc._to_const_object().cell(ci).cell_index(), ci )
 
     ci2 = ly.add_cell( "new_cell_b" )
     self.assertEqual( ly.cells(), 2 )
     self.assertEqual( ly.cell_by_name("new_cell_b"), ci2 )
+    self.assertEqual( sorted([ c.name for c in ly.cells("new*") ]), ['new_cell', 'new_cell_b'] )
     self.assertEqual( ci != ci2, True )
+    lyc = ly.dup()
+    self.assertEqual( sorted([ c.name for c in lyc._to_const_object().cells("new*") ]), ['new_cell', 'new_cell_b'] )
 
     ly.rename_cell( ci2, "x" )
     self.assertEqual( ly.cell_by_name("x"), ci2 )
@@ -918,6 +925,8 @@ def test_8(self):
       tc.append(l.cell(t).name) 
     self.assertEqual(",".join(tc), "c0")
     self.assertEqual(l.top_cell().name, "c0")
+    lc = l.dup()
+    self.assertEqual(lc._to_const_object().top_cell().name, "c0")  # const version
     tc = []
     for t in l.top_cells():
       tc.append(t.name) 
@@ -939,6 +948,11 @@ def test_8(self):
     for t in l.top_cells():
       tc.append(t.name) 
     self.assertEqual(",".join(tc), "c0,c1")
+    tc = []
+    lc = l.dup()
+    for t in lc._to_const_object().top_cells(): # const version
+      tc.append(t.name) 
+    self.assertEqual(",".join(tc), "c0,c1")
 
     c2 = l.create_cell("c1")
     self.assertEqual(c2.name, "c1$1")
diff --git a/testdata/ruby/basic_testcore.rb b/testdata/ruby/basic_testcore.rb
index 6183a77d4..ec073d8b5 100644
--- a/testdata/ruby/basic_testcore.rb
+++ b/testdata/ruby/basic_testcore.rb
@@ -3233,4 +3233,45 @@ def test_81
 
   end
 
+  # constness
+  def test_82
+
+    b = RBA::B::new
+    assert_equal(b.is_const_object, false)
+
+    bc = b._to_const_object
+    assert_equal(bc.is_const_object, true)
+
+    m = ""
+    begin
+      bc.set_str("abc")
+      assert_equal(1, 0)
+    rescue => ex
+      m = ex.to_s
+    end
+
+    assert_equal(m, "Cannot call non-const method on a const reference in B::set_str")
+
+    b = RBA::B::new
+    bc = b
+    assert_equal(b._is_const_object, false)
+    assert_equal(bc._is_const_object, false)
+    b.set_str("abc")
+    bc._to_const_object
+    assert_equal(b._is_const_object, true)  # special
+    assert_equal(bc._is_const_object, true)
+    assert_equal(b.str, "abc")
+    assert_equal(bc.str, "abc")
+
+    bnc = bc._const_cast
+    assert_equal(b._is_const_object, false)  # special
+    assert_equal(bc._is_const_object, false)   # special
+    assert_equal(bnc._is_const_object, false)
+    bnc.set_str("xyz")
+    assert_equal(b.str, "xyz")
+    assert_equal(bc.str, "xyz")
+    assert_equal(bnc.str, "xyz")
+
+  end
+
 end
diff --git a/testdata/ruby/dbLayoutTests2.rb b/testdata/ruby/dbLayoutTests2.rb
index fc1be5ac6..111d0e9f8 100644
--- a/testdata/ruby/dbLayoutTests2.rb
+++ b/testdata/ruby/dbLayoutTests2.rb
@@ -83,14 +83,21 @@ def test_2_Layout
     assert_equal( ly.cell_name(ci), "new_cell" )
 
     assert_equal( ly.cell_by_name("new_cell"), ci )
+    assert_equal( ly.cell(ci).cell_index, ci )
     assert_equal( ly.cells("A*"), [] )
     assert_equal( ly.cell("new_cell").name, "new_cell" )
     assert_equal( ly.cell("x").inspect, "nil" )
+    lyc = ly.dup
+    assert_equal( lyc._to_const_object.cell("new_cell").name, "new_cell" ) 
+    assert_equal( lyc._to_const_object.cell(ci).cell_index, ci ) 
 
     ci2 = ly.add_cell( "new_cell_b" )
     assert_equal( ly.cells, 2 )
     assert_equal( ly.cell_by_name("new_cell_b"), ci2 )
+    assert_equal( ly.cells("new*").collect { |c| c.name }.sort, ['new_cell', 'new_cell_b'] )
     assert_equal( ci != ci2, true )
+    lyc = ly.dup
+    assert_equal( lyc._to_const_object.cells("new*").collect { |c| c.name }.sort, ['new_cell', 'new_cell_b'] )
 
     ly.rename_cell( ci2, "x" )
     assert_equal( ly.cell_by_name("x"), ci2 )
@@ -900,6 +907,9 @@ def test_8
     assert_equal(tc.collect { |s| s.to_s }.join(","), "c0")
     assert_equal(l.top_cell.name, "c0")
     assert_equal(l.top_cells.collect { |t| t.name }.join(","), "c0")
+    lc = l.dup
+    assert_equal(lc._to_const_object.top_cell.name, "c0")
+    assert_equal(lc._to_const_object.top_cells.collect { |t| t.name }.join(","), "c0")
 
     c1 = l.create_cell("c1")
     assert_equal(c1.name, "c1")
diff --git a/testdata/ruby/dbPCells.rb b/testdata/ruby/dbPCells.rb
index 36fa4d5ce..0c192726c 100644
--- a/testdata/ruby/dbPCells.rb
+++ b/testdata/ruby/dbPCells.rb
@@ -900,5 +900,41 @@ def test_12
 
 end
 
+class DBPCellParameterStates_TestClass < TestBase
+
+  def test_1
+
+    ps = RBA::PCellParameterState::new
+
+    ps.value = 17
+    assert_equal(ps.value, 17)
+    ps.value = "u"
+    assert_equal(ps.value, "u")
+
+    ps.visible = true
+    assert_equal(ps.is_visible?, true)
+    ps.visible = false
+    assert_equal(ps.is_visible?, false)
+
+    ps.enabled = true
+    assert_equal(ps.is_enabled?, true)
+    ps.enabled = false
+    assert_equal(ps.is_enabled?, false)
+
+    ps.readonly = true
+    assert_equal(ps.is_readonly?, true)
+    ps.readonly = false
+    assert_equal(ps.is_readonly?, false)
+
+    ps.tooltip = "uvw"
+    assert_equal(ps.tooltip, "uvw")
+
+    ps.icon = RBA::PCellParameterState::InfoIcon
+    assert_equal(ps.icon, RBA::PCellParameterState::InfoIcon)
+
+  end
+
+end
+
 load("test_epilogue.rb")