just return self if trying to evaluate compound values, seeing as we use eval() for variable lookup/eval

lib/less/node/expression.js

@@ -3,7 +3,7 @@ if (typeof(window) === 'undefined') { var tree = require(require('path').join(__
 tree.Expression = function Expression(value) { this.value = value };
 tree.Expression.prototype.eval = function (env) {
     if (this.value.length > 1) {
-        throw new(Error)("can't eval compound expression");
+        return this;
     } else {
         return this.value[0].eval(env); 
     }

lib/less/node/rule.js

@@ -24,7 +24,7 @@ tree.Value.prototype.eval = function (env) {
     if (this.value.length === 1) {
         return this.value[0].eval(env);
     } else {
-        throw new(Error)("trying to evaluate compound value");
+        return this;
     }
 };
 tree.Value.prototype.toCSS = function (env) {