don't re-eval ruleset

lib/less/tree/ruleset.js

@@ -7,6 +7,8 @@ tree.Ruleset = function (selectors, rules) {
 };
 tree.Ruleset.prototype = {
     eval: function (env) {
+        if (this.evaled) { return this }
+
         var rules = [];
 
         this.rules.forEach(function (rule) {
@@ -17,6 +19,8 @@ tree.Ruleset.prototype = {
             }
         });
         this.rules = rules;
+        this.evaled = true;
+
         return this;
     },
     match: function (args) {