diff --git a/lib/6to5/traverse/index.js b/lib/6to5/traverse.js
similarity index 77%
rename from lib/6to5/traverse/index.js
rename to lib/6to5/traverse.js
index 67633d20ef..0938c4d301 100644
--- a/lib/6to5/traverse/index.js
+++ b/lib/6to5/traverse.js
@@ -1,23 +1,30 @@
-var _ = require("lodash");
+module.exports = traverse;
 
-var traverse = module.exports = function (parent, callbacks, blacklistTypes) {
+var Scope = require("./scope");
+var t     = require("./types");
+var _     = require("lodash");
+
+function traverse(parent, callbacks, opts) {
   // falsy node
   if (!parent) return;
 
   // array of nodes
   if (_.isArray(parent)) {
     _.each(parent, function (node) {
-      traverse(node, callbacks, blacklistTypes);
+      traverse(node, callbacks, opts);
     });
     return;
   }
 
   // unknown node type to traverse
-  var keys = traverse.VISITOR_KEYS[parent.type];
+  var keys = t.VISITOR_KEYS[parent.type];
   if (!keys) return;
 
+  opts = opts || {};
+  if (_.isArray(opts)) opts = { blacklist: opts };
+
   // blacklist these node types from being traversed
-  blacklistTypes = blacklistTypes || [];
+  var blacklistTypes = opts.blacklist || [];
 
   // normalise callbacks
   if (_.isFunction(callbacks)) callbacks = { enter: callbacks };
@@ -37,12 +44,13 @@ var traverse = module.exports = function (parent, callbacks, blacklistTypes) {
 
       // replace node
       var maybeReplace = function (result) {
+        if (result === false) return;
         if (result != null) node = obj[key] = result;
       };
 
       // enter
       if (callbacks.enter) {
-        result = callbacks.enter(node, parent, obj, key);
+        result = callbacks.enter(node, parent, opts);
 
         // stop iteration
         if (result === false) return;
@@ -51,11 +59,13 @@ var traverse = module.exports = function (parent, callbacks, blacklistTypes) {
       }
 
       // traverse node
-      traverse(node, callbacks, blacklistTypes);
+      var opts2 = _.clone(opts);
+      if (t.isScope(node)) opts2.scope = new Scope(opts.scope, node);
+      traverse(node, callbacks, opts2);
 
       // exit
       if (callbacks.exit) {
-        maybeReplace(callbacks.exit(node, parent, obj, key));
+        maybeReplace(callbacks.exit(node, parent, opts));
       }
     };
 
@@ -71,8 +81,6 @@ var traverse = module.exports = function (parent, callbacks, blacklistTypes) {
   });
 };
 
-traverse.VISITOR_KEYS = require("./visitor-keys");
-
 traverse.removeProperties = function (tree) {
   var clear = function (node) {
     delete node.tokens;
@@ -121,7 +129,7 @@ traverse.hasType = function (tree, type, blacklistTypes) {
         has = true;
         return false;
       }
-    }, blacklistTypes);
+    }, { blacklist: blacklistTypes });
   }
 
   return has;
diff --git a/lib/6to5/types/index.js b/lib/6to5/types/index.js
index 8fdd0e2f01..31533075e7 100644
--- a/lib/6to5/types/index.js
+++ b/lib/6to5/types/index.js
@@ -6,7 +6,9 @@ var t = exports;
 
 //
 
-_.each(traverse.VISITOR_KEYS, function (keys, type) {
+t.VISITOR_KEYS = require("./visitor-keys");
+
+_.each(t.VISITOR_KEYS, function (keys, type) {
   t["is" + type] = function (node) {
     return node && node.type === type;
   };
@@ -14,7 +16,7 @@ _.each(traverse.VISITOR_KEYS, function (keys, type) {
 
 //
 
-t.BUILDER_KEYS = _.defaults(require("./builder-keys"), traverse.VISITOR_KEYS);
+t.BUILDER_KEYS = _.defaults(require("./builder-keys"), t.VISITOR_KEYS);
 
 _.each(t.BUILDER_KEYS, function (keys, type) {
   t[type[0].toLowerCase() + type.slice(1)] = function () {
@@ -92,6 +94,37 @@ t.toBlock = function (node, parent) {
   return t.blockStatement(node);
 };
 
+t.getIds = function (node) {
+  var search = [node];
+  var ids  = [];
+
+  while (search.length) {
+    var id = search.shift();
+
+    if (t.isIdentifier(id)) {
+      ids.push(id.name);
+    } else if (t.isArrayPattern(id)) {
+      _.each(id.elements, function (elem) {
+        search.push(elem);
+      });
+    } else if (t.isAssignmentExpression(id)) {
+      search.push(id.left);
+    } else if (t.isObjectPattern(id)) {
+      _.each(id.properties, function (prop) {
+        search.push(prop.value);
+      });
+    } else if (t.isVariableDeclaration(id)) {
+      search = search.concat(id.declarations);
+    } else if (t.isVariableDeclarator(id)) {
+      search.push(id.id);
+    } else {
+      throw new Error("unknown node " + id.type);
+    }
+  }
+
+  return ids;
+};
+
 t.inherits = function (child, parent) {
   child.loc   = parent.loc;
   child.end   = parent.end;
@@ -119,6 +152,10 @@ t.needsWhitespace = function (node, expression) {
     return true;
   }
 
+  if (t.isCallExpression(node) && t.isFunction(node.callee)) {
+    return true;
+  }
+
   if (expression) return false;
 
   //
@@ -192,6 +229,11 @@ t.needsParans = function (node, parent) {
     }
   }
 
+  // (class {});
+  if (t.isClassExpression(node) && t.isExpressionStatement(parent)) {
+    return true;
+  }
+
   if (t.isSequenceExpression(node)) {
     if (t.isForStatement(parent)) {
       // Although parentheses wouldn't hurt around sequence
diff --git a/lib/6to5/traverse/visitor-keys.json b/lib/6to5/types/visitor-keys.json
similarity index 100%
rename from lib/6to5/traverse/visitor-keys.json
rename to lib/6to5/types/visitor-keys.json