code: func (e *ErrorClass) Contains(err error, opts ...EquivalenceOption) bool {
  	if err == nil {
  		return false
  	}
  	cast, ok := err.(*Error)
  	if !ok {
  		return findSystemErrorClass(err).Is(e)
  	}
  	if cast.class.Is(e) {
  		return true
  	}
  	if combineEquivOpts(opts)&IncludeWrapped == 0 {
  		return false
  	}
  	return e.Contains(cast.err, opts...)
  }
docs: // Contains returns whether or not the receiver error class contains the given
  // error instance.