One principle used, concerning which there is some disagreement in practice among mathematicians, is that the double bar '=' taken as the sign of identity.

위 문장에서 One principle used, concerning which there is some disagreement in practice among mathematicians 부분은 어떻게 해석해야 하나요? 부사절은 아닌 것 같은데... 특히 concerning which 부분이 헛갈립니다.