sub parse_expr {
    my($value, @stack);
    while(1) {
        $value = parse_value or die "Parse error";
        my $op = parse_operator or last;
        $value = process(\@stack, $value, $op);  # intermediate processing
        push @stack, { value => $value, op => $op };  # store for the next round 
    }
    $value = process(\@stack, $value);  # final processing
    return $value;
}