Fix field reference

Author: mattn

ops.go

@@ -513,14 +513,14 @@ func doLet(env *Env, node *Node) (*Node, error) {
 			}
 			switch curr.car.car.t {
 			case NodeCell:
-				obj, err := eval(env, curr.car.car.cdr.cdr.car)
+				obj, err := eval(env, curr.car.car.cdr.car)
 				if err != nil {
 					return nil, err
 				}
 				if obj.t == NodeCell && obj.car != nil && obj.car.t == NodeGoValue {
 					obj = obj.car
 				}
-				fld := obj.v.(reflect.Value).Elem().FieldByName(curr.car.car.cdr.car.v.(string))
+				fld := obj.v.(reflect.Value).Elem().FieldByName(curr.car.car.cdr.cdr.car.v.(string))
 				fld.Set(vv.car.v.(reflect.Value))
 			case NodeIdent:
 				scope.vars[curr.car.car.v.(string)] = vv