As is documented at https://golang.org/pkg/errors/#As.

Is is documented at https://golang.org/pkg/errors/#Is.

New returns an error with the given description that can be tagged.

Newf returns an error with the given formatted description that can be tagged.

Tag retrieves the value of the tag named by the key.

Unwrap is documented at https://golang.org/pkg/errors/#Unwrap.

Error is an error implementation that supports tagging and wrapping.