code: func ExpectCompareFailed(c *check.C, err error) {
  	c.Assert(trace.IsCompareFailed(err), check.Equals, true, check.Commentf("expected CompareFailed, got %T %v at %v", trace.Unwrap(err), err, string(debug.Stack())))
  }
docs: // ExpectCompareFailed expects compare failed error