auto db = Sqlite3::Db("filename");

return Ev::lift().then([db]() {
    /* This could block if a transaction is currently in-flight.  */
    return db.transact();
}).then([](Sqlite3::Tx tx) {
    auto res = tx.query("SELECT (a, b) FROM tablename WHERE id=:ID")
             .bind(":ID", 42)
             .execute()
             ;
    /* `res` is a single-pass sequence!
     * It has an iterator type that is an input iterator
     * (can only dereference and advance, but copying
     * does not create a distinct state).
     */
    for (auto r : res) {
        auto a = r.column<int>(0);
        auto b = r.column<std::string>(1);
        process(a, b);
    }
    /* Sqlite3::Tx cannot be copied, but can be moved.
     * Once it has been destructed, it automatically
     * rolls back and lets other greenthreads execute
     * their transactions.
     *
     * In order to actually commit the transaction,
     * you have to call Sqlite3::Tx::commit on it.
     * This atomically writes the changes to disk,
     * destroys the Tx object (meaning it is in an
     * invalid, moved-from state where it cannot be
     * used for queries) and lets other greenthreads
     * waiting to transact continue.
     */
    tx.commit();
    
    return Ev::lift();
});