Wrap an external customized error end up as a rootErr ?

[westwin]

according to the document of Wrap(),

// Wrap adds additional context to all error types while maintaining the type of the original error.
//
// This method behaves differently for each error type. For root errors, the stack trace is reset to the current
// callers which ensures traces are correct when using global/sentinel error values. Wrapped error types are simply
// wrapped with the new context. For external types (i.e. something other than root or wrap errors), this method
// attempts to unwrap them while building a new error chain. If an external type does not implement the unwrap
// interface, it flattens the error and creates a new root error from it before wrapping with the additional
// context.

it says for external types, it attempts to unwrap the (the unwrap interface), but I didn't find the Unwrap() is called

btw, what is the best practice to wrap an external error, say
type MyError struct {
Code int // error code
}