code: func (f *Failure) Backtrace() string {
  	tr := f.UserTrace
  	if tr == nil {
  		tr = f.FailTrace
  	}
  	return tr.String() + "Error: " + f.Message
  }
docs: // Backtrace returns a user-friendly error message describing the stack of
  // calls that led to this error.
  //
  // If fail(...) was called with a custom stack trace, this trace is shown here.
  // Otherwise the trace of where fail(...) happened is used.