code: func (src InMemorySource) Response(request *ocsp.Request) ([]byte, http.Header, error) {
  	response, present := src[request.SerialNumber.String()]
  	if !present {
  		return nil, nil, ErrNotFound
  	}
  	return response, nil, nil
  }
docs: // Response looks up an OCSP response to provide for a given request.
  // InMemorySource looks up a response purely based on serial number,
  // without regard to what issuer the request is asking for.