my %contents = (
    1 => [ 'Introduction', undef ],
    2 => [ 'Design', {
        1 => [ 'Wheels', undef ],
        2 => [ 'Frame', undef ],
        3 => [ 'Electronics', {
            1 => [ 'Dashboard', undef ],
            2 => [ 'Fuel Injection', undef ],
        },
        4 => [ 'Petrol Tank', undef ],
    },
    3 => [ 'Summary', undef ],
  );