lv1expr : <leftop:lv2expr addsub lv1expr>  {

        my @ops = @{$item[1]};

        my $res = shift @ops;

        while (@ops) {

                my ($op, $num) = (shift@ops, shift@ops);

                if ($op eq '+') { $res+= $num }

                else { $res -= $num; }

        }

        $return = $res;

    }

# ...

addsub: /[-+]/