** We can declare a method on non-struct types too */

* The error type is a built-in interface similar to fmt.Stringer type error interface { Error() string } */

Since `error` is an interface, we can use arbitrary data structures

** * We can use type assertion like * * if nerr, ok := err.(NetworkError); ok && nerr.Temporate() { ... } */

Worker keps a channel of reqs, some load tracking data

`error` type is an interface type error interface { Error() string } We can implement our own error type