code: func (rl *respLogger) StacktraceWhen(pred StacktracePred) *respLogger {
  	rl.logStacktracePred = pred
  	return rl
  }
docs: // StacktraceWhen sets the stacktrace logging predicate, which decides when to log a stacktrace.
  // There's a default, so you don't need to call this unless you don't like the default.