Correctly handle rules that start with a parenthesis (should not be...

Correctly handle rules that start with a parenthesis (should not be interpreted as function, but as operator)

rules_shipping_advanced.php

@@ -194,8 +194,8 @@ class ShippingRule_Advanced extends ShippingRule {
 		$is_condition = false;
 		$is_assignment = false;
 		
-		$stack = array ();  // 1)
-		$prev_token_operator = false;
+		$stack = array ();  // 1)/
+		$prev_token_operator = true;
 		$function_args = array();
 		$out_stack = array();
 		foreach ($atoms as $a) { // 2)