DefaultStacktracePred is the default implementation of StacktracePred.

LogOf returns the logger hiding in w.

NewLogged turns a normal response writer into a logged response writer.

StatusIsNot returns a StacktracePred which will cause stacktraces to be logged for any status *not* in the given list.

Unlogged returns the original ResponseWriter, or w if it is not our inserted logger.

StacktracePred returns true if a stacktrace should be logged for this status.