init do
    `create unique index on users(id)`
end

process do
    transaction tx1 read_committed do
        let count_1 := `select count(age) from users where id = 1`
        if count_1 >= 1 do
            `update users set age := 21 where id = 1`
        else
            `insert into users(id, age) values (1, 21)`
        end
    end
end

process do
    transaction tx2 read_committed do
        let count_2 := `select count(age) from users where id = 1`
        if count_2 >= 1 do
            `update users set age := 21 where id = 1`
        else
            `insert into users(id, age) values (1, 20)`
        end
    end
end

property eventually(`select id, age from users` in {
    (1, 20), (1, 21)
})