CD[][]-function

[Petr Tretyakov]

I try to study xAct by the video lessons of 
Konstantinos Anagnostopoulos in YouTube. I have a silly question: on about 01:03:00 of the first lesson he explain the difference  between usual and covariant derivatives i.e. functions PD[][] and CD[][]. I try to follow all actions on video and find that no such function CD[][]. Moreover if jump to the 0:09:50 moment of video, where he demonstrate all commands and functions of xAct package, we see that PD[][] exist, but CD[][] -- no such function in the list. There exist only CDIndexQ, CovD, CovDOfMetric, and so on. May be I missed something, but I can not to see in my Wolfram system nothing similar on 01:03:20 which is just standard notation for covariant derivative of vector.

[Yuqian Zhao]

In general, you should have defined 'CD' when you defined the metric. For example, he used DefMetric[-1, g[-\mu,-\nu], CD] at about 00:20:00 of the video.

[Petr Tretyakov]

Thank you!