diff --git a/lib/carto/renderer.js b/lib/carto/renderer.js
index 98370d0..6af4be7 100644
--- a/lib/carto/renderer.js
+++ b/lib/carto/renderer.js
@@ -60,13 +60,15 @@ carto.Renderer.prototype.render = function render(m, callback) {
         });
         var rules = inheritRules(matching, env);
         var sorted = sortStyles(rules, env);
+
         _(sorted).each(function(rule) {
             var style = new tree.Style(l.name, rule.attachment, rule);
             if (style) {
                 l.styles.push(style.name);
 
+                var xml = style.toXML(env);
                 // env.effects can be modified by this call
-                output.push(style.toXML(env));
+                output.push(xml);
             }
         });
         if (l.styles.length) {
@@ -170,7 +172,8 @@ function addRules(current, definition, existing) {
                     k++;
                 }
             }
-        } else if (updatedFilters === null) {
+        } else if (updatedFilters === null &&
+            current[k].zoom === definition.zoom) {
             // Filters can be added, but they don't change the
             // filters. This means we don't have to split the
             // definition.
@@ -213,6 +216,7 @@ function inheritRules(definitions, env) {
             byAttachment[attachment].push(current[k]);
         }
     }
+    // console.log(JSON.stringify(result, null, 4));
 
     if (env.benchmark) console.warn('Inheritance time: ' + ((new Date() - inheritTime)) + 'ms');
 
diff --git a/lib/carto/tree/definition.js b/lib/carto/tree/definition.js
index bfe4fd0..446d678 100644
--- a/lib/carto/tree/definition.js
+++ b/lib/carto/tree/definition.js
@@ -65,13 +65,13 @@ tree.Definition.prototype.appliesTo = function(id, classes) {
 };
 
 tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
-    var xml = '  <Rule>\n';
+    var xml = '  <Rule>\n', key;
     xml += tree.Zoom.toXML(zoom).join('');
     xml += this.filters.toXML(env);
 
     // Sort symbolizers by the index of their first property definition
     var sym_order = [], indexes = [];
-    for (var key in symbolizers) {
+    for (key in symbolizers) {
         indexes = [];
         for (var prop in symbolizers[key]) {
             indexes.push(symbolizers[key][prop].index);
@@ -87,6 +87,10 @@ tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
         return v[0];
     });
 
+    function capitalize(str) {
+        return str[1].toUpperCase();
+    }
+
     for (var i = 0; i < sym_order.length; i++) {
         var attributes = symbolizers[sym_order[i]];
         var symbolizer = sym_order[i].split('/').pop();
@@ -106,13 +110,11 @@ tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
         }
 
         var name = symbolizer.charAt(0).toUpperCase() +
-               symbolizer.slice(1).replace(/\-./, function(str) {
-                   return str[1].toUpperCase();
-               }) + 'Symbolizer';
+               symbolizer.slice(1).replace(/\-./, capitalize) + 'Symbolizer';
 
         var selfclosing = true, tagcontent;
         xml += '    <' + name + ' ';
-        for (var key in attributes) {
+        for (key in attributes) {
             if (symbolizer === 'map') env.error({
                 message: 'Map properties are not permitted in other rules',
                 index: attributes[key].index,
@@ -140,6 +142,8 @@ tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
     return xml;
 };
 
+// Take a zoom range of zooms and 'i', the index of a rule in this.rules,
+// and finds all applicable symbolizers
 tree.Definition.prototype.collectSymbolizers = function(zooms, i) {
     var symbolizers = {}, child;
 
diff --git a/lib/carto/tree/filterset.js b/lib/carto/tree/filterset.js
index 10d9383..0da1c01 100644
--- a/lib/carto/tree/filterset.js
+++ b/lib/carto/tree/filterset.js
@@ -46,6 +46,8 @@ Object.defineProperty(tree.Filterset.prototype, 'cloneWith', {
         var additions;
         for (var id in other) {
             var status = this.addable(other[id]);
+            // status is true, false or null. if it's null we don't fail this
+            // clone nor do we add the filter.
             if (status === false) {
                 return false;
             }
@@ -58,7 +60,9 @@ Object.defineProperty(tree.Filterset.prototype, 'cloneWith', {
 
         // Adding the other filters doesn't make this filterset invalid, but it
         // doesn't add anything to it either.
-        if (!additions) return null;
+        if (!additions) {
+            return null;
+        }
 
         // We can successfully add all filters. Now clone the filterset and add the
         // new rules.
@@ -66,7 +70,7 @@ Object.defineProperty(tree.Filterset.prototype, 'cloneWith', {
 
         // We can add the rules that are already present without going through the
         // add function as a Filterset is always in it's simplest canonical form.
-        for (var id in this) {
+        for (id in this) {
             clone[id] = this[id];
         }
 
@@ -79,20 +83,21 @@ Object.defineProperty(tree.Filterset.prototype, 'cloneWith', {
     }
 });
 
-/**
- * Returns true when the new filter can be added, false otherwise.
- */
+// Returns true when the new filter can be added, false otherwise.
+// It can also return null, and on the other side we test for === true or
+// false
 Object.defineProperty(tree.Filterset.prototype, 'addable', {
     enumerable: false,
     value: function(filter) {
-        var key = filter.key, value = filter.val;
+        var key = filter.key,
+            value = filter.val;
 
         switch (filter.op) {
             case '=':
                 if (key + '=' in this) return (this[key + '='].val != value) ? false : null;
                 if (key + '!=' + value in this) return false;
-                if (key + '>' in this  && this[key + '>'].val >= value) return false;
-                if (key + '<' in this  && this[key + '<'].val <= value) return false;
+                if (key + '>' in this && this[key + '>'].val >= value) return false;
+                if (key + '<' in this && this[key + '<'].val <= value) return false;
                 if (key + '>=' in this && this[key + '>='].val > value) return false;
                 if (key + '<=' in this && this[key + '<='].val < value) return false;
                 return true;
@@ -103,8 +108,8 @@ Object.defineProperty(tree.Filterset.prototype, 'addable', {
             case '!=':
                 if (key + '=' in this) return (this[key + '='].val == value) ? false : null;
                 if (key + '!=' + value in this) return null;
-                if (key + '>' in this  && this[key + '>'].val >= value) return null;
-                if (key + '<' in this  && this[key + '<'].val <= value) return null;
+                if (key + '>' in this && this[key + '>'].val >= value) return null;
+                if (key + '<' in this && this[key + '<'].val <= value) return null;
                 if (key + '>=' in this && this[key + '>='].val > value) return null;
                 if (key + '<=' in this && this[key + '<='].val < value) return null;
                 return true;
@@ -150,11 +155,11 @@ Object.defineProperty(tree.Filterset.prototype, 'addable', {
 Object.defineProperty(tree.Filterset.prototype, 'add', {
     enumerable: false,
     value: function(filter) {
-        var key = filter.key;
+        var key = filter.key, id;
 
         switch (filter.op) {
             case '=':
-                for (var id in this) {
+                for (id in this) {
                     if (this[id].key == key) {
                         delete this[id];
                     }
@@ -174,7 +179,7 @@ Object.defineProperty(tree.Filterset.prototype, 'add', {
                 // If there are other filters that are also >
                 // but are less than this one, they don't matter, so
                 // remove them.
-                for (var id in this) {
+                for (id in this) {
                     if (this[id].key == key && this[id].val <= filter.val) {
                         delete this[id];
                     }
@@ -183,7 +188,7 @@ Object.defineProperty(tree.Filterset.prototype, 'add', {
                 break;
 
             case '>=':
-                for (var id in this) {
+                for (id in this) {
                     if (this[id].key == key && this[id].val < filter.val) {
                         delete this[id];
                     }
@@ -199,7 +204,7 @@ Object.defineProperty(tree.Filterset.prototype, 'add', {
                 break;
 
             case '<':
-                for (var id in this) {
+                for (id in this) {
                     if (this[id].key == key && this[id].val >= filter.val) {
                         delete this[id];
                     }
@@ -208,7 +213,7 @@ Object.defineProperty(tree.Filterset.prototype, 'add', {
                 break;
 
             case '<=':
-                for (var id in this) {
+                for (id in this) {
                     if (this[id].key == key && this[id].val > filter.val) {
                         delete this[id];
                     }
diff --git a/test/rendering/sharedclass.mml b/test/rendering/sharedclass.mml
index e73b1bd..cb223de 100644
--- a/test/rendering/sharedclass.mml
+++ b/test/rendering/sharedclass.mml
@@ -17,7 +17,7 @@
     },
     {
       "Datasource": {
-        "file": "http://tilemill-data.s3.amazonaws.com/natural-earth-10m-1.3.0/urban_areas.zip",
+        "file": "http://tilemill-data.s3.amazonaws.com/world_borders_merc.zip",
         "type": "shape"
       },
       "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",