This is a Go library providing error handling primitives with stack trace capture, wrapping, and cause retrieval. The primary functions are `New`, `Errorf`, `Wrap`, `Wrapf`, `WithStack`, `WithMessage`, and `Cause`. It implements the `fmt.Formatter` interface for detailed error formatting and includes compatibility with Go 1.13's `errors.Is`, `As`, and `Unwrap`. The library is self-contained with no external dependencies beyond the standard library. It is in maintenance mode as the Go standard library has adopted similar functionality, but it remains widely used for its stack trace capabilities. The architecture is a single package with a few types: `fundamental`, `withStack`, `withMessage`, and `Frame`/`StackTrace` for stack representation. The code is well-tested, well-documented, and demonstrates idiomatic Go patterns.