===== Sentinel errors =====

==== What is it? ====
A **sentinel error** is a package-level variable used as a fixed, recognizable error value.

Example:
<code go>
var ErrNotFound = errors.New("not found")
</code>

==== What is it used for? ====
  * Let callers check for a specific error using equality (`err == ErrNotFound`)
    or (preferably) `errors.Is(err, ErrNotFound)` when wrapping is involved.

==== Example ====
<code go>
package store

import "errors"

var ErrNotFound = errors.New("not found")
</code>

==== Notes / pitfalls ====
  * Sentinel errors are simple but can be too generic for large systems.
  * Prefer typed errors when you need extra context (resource/id).
  * If you wrap errors, use `errors.Is` to match the sentinel.

==== Hard words (English) ====
  * **sentinel** /ˈsentɪnəl/: dấu hiệu nhận diện/cột mốc
  * **recognizable** /ˈrekəɡnaɪzəbəl/: dễ nhận ra
  * **generic** /dʒəˈnerɪk/: chung chung