debug

type Debug interface {
	// Printf prints, with a short file:line-number prefix
	Printf(format string, args ...interface{})
	// Writer returns the writer being written to, which may be
	// `io.Discard` if no debug output is requested.
	Writer() io.Writer

	// Output allows tweaking the calldepth used for figuring
	// out which Go source file location is the interesting one,
	// i.e., which is included in the debug message. Useful for
	// setting up local helper methods.
	Output(calldepth int, s string) error
}