small compatibility fix for prototype.js

lib/less/parser.js

@@ -1934,7 +1934,7 @@ less.Parser = function Parser(env) {
                     // at last, we have the complete match now. move forward, 
                     // convert @{var}s to tree.Variable(s) and return:
                     skipWhitespace(length);
-                    for (var k in name) {
+                    for (var k=0; k < name.length; k++) {
                         if (name[k].charAt(0) === '@') {
                             name[k] = new tree.Variable('@' + name[k].slice(2, -1), 
                                 index[k], env.currentFileInfo);