diff --git a/common/content/commands.js b/common/content/commands.js
index 99daaf8b..14a1df86 100644
--- a/common/content/commands.js
+++ b/common/content/commands.js
@@ -264,7 +264,8 @@ var Command = Class("Command", {
     },
     argsPrototype: Class.memoize(function () update([],
             array(this.options).filter(function (opt) opt.default !== undefined)
-                               .map(function (opt) [opt.names[0], opt.default]).toObject(),
+                               .map(function (opt) [opt.names[0], Class.Property(Object.getOwnPropertyDescriptor(opt, "default"))])
+                               .toObject(),
             {
                 __iterator__: function () array.iterItems(this),
                 command: this,
diff --git a/common/content/mappings.js b/common/content/mappings.js
index ce224ca4..48533dbc 100644
--- a/common/content/mappings.js
+++ b/common/content/mappings.js
@@ -498,10 +498,7 @@ var Mappings = Module("mappings", {
                             description: "Mapping group to which to add this mapping",
                             type: CommandOption.STRING,
                             get default() io.sourcing && io.sourcing.mapHive || mappings.userHive,
-                            completer: function (context) {
-                                context.keys = { text: "name", description: function (h) h.description || h.filter };
-                                context.completions = mappings.allHives.filter(function (h) h.name != "builtin");
-                            },
+                            completer: function (context) completion.mapGroup(context),
                             validator: Option.validateCompleter
                         },
                         {
@@ -608,10 +605,8 @@ var Mappings = Module("mappings", {
             function (args) {
                 dactyl.assert(args.length <= 2, "Trailing characters");
 
-                if (args.length == 0) {
-                    throw FailedAssertion("Not implemented");
-                    return;
-                }
+                if (args.length == 0)
+                    return void completion.listCompleter("mapGroup", "");
 
                 let name = Option.dequote(args[0]);
                 let hive = mappings.getHive(name);
@@ -645,6 +640,20 @@ var Mappings = Module("mappings", {
             {
                 argCount: "*",
                 bang: true,
+                completer: function (context, args) {
+                    if (args.length == 1)
+                        completion.mapGroup(context);
+                    else {
+                        Option.splitList(context.filter);
+                        context.advance(Option._splitAt);
+
+                        context.compare = CompletionContext.Sort.unsorted
+                        context.completions = [
+                            [buffer.uri.host, "Current Host"],
+                            [buffer.uri.spec, "Current Page"]
+                        ];
+                    }
+                },
                 keepQuotes: true,
                 options: [
                     {
@@ -748,6 +757,11 @@ var Mappings = Module("mappings", {
                                [mode.name.toLowerCase()]);
     },
     completion: function () {
+        completion.mapGroup = function mapGroup(context, modes) {
+            context.title = ["Map group"];
+            context.keys = { text: "name", description: function (h) h.description || h.filter };
+            context.completions = mappings.allHives.filter(function (h) h.name != "builtin");
+        };
         completion.userMapping = function userMapping(context, modes) {
             // FIXME: have we decided on a 'standard' way to handle this clash? --djk
             modes = modes || [modules.modes.NORMAL];
diff --git a/common/content/options.js b/common/content/options.js
index 4211af66..d1780c11 100644
--- a/common/content/options.js
+++ b/common/content/options.js
@@ -1250,11 +1250,14 @@ var Options = Module("options", {
             case "regexpmap":
                 let vals = Option.splitList(context.filter);
                 let target = vals.pop() || "";
+
                 let [count, key, quote] = Commands.parseArg(target, /:/, true);
                 let split = Option._splitAt;
+
                 extra.key = Option.dequote(key);
                 extra.value = count < target.length ? Option.dequote(target.substr(count + 1)) : null;
                 extra.values = opt.parse(vals.join(","));
+
                 Option._splitAt = split + (extra.value == null ? 0 : count + 1);
                 break;
             }
diff --git a/common/modules/base.jsm b/common/modules/base.jsm
index d8801c9e..13c13471 100644
--- a/common/modules/base.jsm
+++ b/common/modules/base.jsm
@@ -708,7 +708,7 @@ else
  * @param {Object} desc The property descriptor.
  */
 Class.Property = function Property(desc) update(
-    Object.create(Property.prototype), desc);
+    Object.create(Property.prototype), desc || { configurable: true, writable: true });
 Class.Property.prototype.init = function () {};
 /**
  * Extends a subclass with a superclass. The subclass's
@@ -1152,7 +1152,10 @@ update(iter, {
     toObject: function toObject(iter) {
         let obj = {};
         for (let [k, v] in iter)
-            obj[k] = v;
+            if (v instanceof Class.Property)
+                Object.defineProperty(obj, k, v.init(k) || v);
+            else
+                obj[k] = v;
         return obj;
     },
 
@@ -1283,7 +1286,12 @@ var array = Class("array", Array, {
      */
     toObject: function toObject(assoc) {
         let obj = {};
-        assoc.forEach(function ([k, v]) { obj[k] = v; });
+        assoc.forEach(function ([k, v]) {
+            if (v instanceof Class.Property)
+                Object.defineProperty(obj, k, v.init(k) || v);
+            else
+                obj[k] = v;
+        });
         return obj;
     },